Bolter firepower analysis thread

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Bolter firepower analysis thread

Post by Connor MacLeod » 2008-05-01 03:39am

This has been brewing at the back of my mind for some time now. I admit when I actually starrted doing the analysis bit I was at a complete loss as to how to quantify the explosive effects of the almighty bolter. The kinetic/imapct effects were simpler (Recoil, or by measuring the mass of the projectile and so on and so forth.) I knew some about that, but precisely zippo about high explosives.

It would be "roughly" possible to get an estimate by assuming a projectile mass and internal composition of explosive to get an estimate, but it would be just an estimate, rather than a measurement of effects. So no really accurate attempt at quantifying bolters was possible. Until Mythbusters at least.

A number of Mythbusters episodes have centered around the use of explosives ant the human body. Two notable ones involved grenades and one that involved exploding pens. With these, we've at last got quantiative evidencee to measure bolters by.

Generally speaking, I shouldn't have to describe the various examples of bolter effects - everyone is well acquainted with the effects of bolter shells on the body (I've documented many as ti is.) Some shells blow large fist or head sized holes in the body (human or space marine, depending on source) - which is roughly between 10-20 cm diameter holes. Others blow heads, torsos, or even entire bodies apart. Some vaporize or cauterize, but I won't address those here, as they're easily calced.

The first calc (and the easier) is the Grenade on a ballistics gel body (see link above.) It was a standard fragmentation grenade, and it managed to basically blow apart the torso. Knowing that, we simply need a hand grenade to go by. For convenience sake I'll use the US M61 and M67 Grenades. The former has 6.5 ounces of Composition B, and the latter has 5.5 ounces. We'll call it roughly between 156-185 grams of Composition B, which is roughly 1.35 times more powerful than TNT - or 5.7 MJ per kg. This yields between 900 and 1000 kilojoules of energy (though of course, energy is only patr of the effect ove a conventional explosive - the blast is the more damaging part.)

So, given that, and given we know some bolter shells can blow torsos apart, ,we can conjecture that a bolter round (or several - 3rd edition says that bolters will fire 3-4 shells per trigger pull) is roughly equal to a Grenade.

There are some considerations. Since its an omnidirectional blast, technically a grenade won't direct ALL its energy to the person - part of it will probably hit the ground and may reflect up, but its at least half. Also, its a fragmentation grenade,a nd not all bolter shells are frag (only the metla storm are.) REgular grenades would be more equal to a "concussion" grenade, and thus might not be as effective. Both situations are probably mitigated somewhat by internal detonation as well, though.

I should note that an earlier episode dealing with nitrogylcerine patches featured a similar case where a ballistics gel torso was obliterated by high explosives and nitro poured into a small (several inch diameter) cavity in the gel dummy's chest (the explosive, which remained unidentified, was by my estimates only a bit smaller then the hole.) and by my estimates provided similar results above.

Now, that one is quicker, but less precise. The "exploding pen" (see liknk above) is harder, but was done with multiple examples.

The first example used a "regular" sized pen. The Mythbusters measured its internal volume at around 3 cubic centimeters. It also put a "grapefruit" sized hole in the target (which is roughly fist or head shaped)

We don't hear the type fo explosive used, but we can guess.

TNT has a density of 1.654 g/cm^3, and obviously a RE of 1.0 (since its the baseline.)

RDX has a density of 1.82 g/cm^3 and a RE of 1.6.

The (currently) most powerful explosive I am aware of is octanitrocubane:

it has a RE of 2.7 and a density of 2 g/cm^3.

By regular TNT, the pen would carry around 5 grams of TNT, 5.4 grams of RDX, or 6 grams of ONC. The energy equivalents would be 21 kilojoules for TNT, 36.2 kilojoules for RDX, and 68 kilojoules for ONC.

For the most parrt I'm betting it was something better than TNT (it sounded like it might be restricted or secret) so ti would probably be between RDX and ONC in terms of power.

The second one was actually easier to measure. Jamie said that it had the "better part" of as tick of dynamite in it. "Better part" which argues more than half (backup definition here.)

A stick of dynamite is about 2000 BTU, which is confirmed here. Dynamite is also noted to have around 5000 BTU per pound here, and an average (20 cm long, 2.5 cm diameter) stick is probably around 200 grams, so it works out. Half of that, would be around 1000 BTU, or a little over one megajoule (at least.)

So, we can say between 1-2 megajoules to blow up a torso with the exploding pen, although again its an omnidirectional blast which will affect things (though by no more than half.).

Its also interesting to note that yield-wise, it was said that the other pen (with the unknown explosive) had 1/6 the yield of the second test, suggesting it was closer to 100-200 kilojoules of TNT to blow a fist/head sized hole in the target.

Note that while the Mythbusters weren't "convinced" by this blast, it probably would match what is described in 40K sources for bolters blasting torsos.

The last pen explosion isn't quantified, but was considerably bigger than the second or first one, and it also totally obliterated the torso. Going by estimates, I'd say its at least 50% longer and perhaps twice the diameter of ths second pen, so again its probably 6x larger. This would suggest that it was explosively comparable to 1-2 kg of TNT.

I would not generally consider the last one as being analogous to bolter ounds as the blast effects observed in the video seem to encompass a substantially greater volume than the earlier two detonations,a nd bolters aren't quite THAT nasty (IE they're not area affect wepaons like grenade or missile launchers.) so we can probably rule that one out.

Taking all of the above into context, its fair to say that bolter shells are probably equal to many tens or hundreds of grams of TNT, given comparison of observed effects.

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Post by Connor MacLeod » 2008-05-01 03:50am

Oh, as an addendum:

I mentioned erlier that before this the best means of estimating Bolter firepower was to gauge the internal volume of the shell.

We know bolter shells are about .75 calibre, so they are roughly 1.9 cm in diameter. The length of the shell appears to be roughly 3-4x the diameter as well, which would be around 5.7 and 7.6 cm. Judging by the cutaways of bolter shells, I'd esitmate no more than half - more probably a third, of the internal volume is dedicated to explosive (the rest is propellant and casing.) The total volume of the shell would be roughly between 17 and 23 cubic cm by estimate, so the internal volume of the explosive is probably between 8-12 cubic centimeters (for half) and 6-8 cubic centimeters (for a third)

With the densities/REFs outlined above, we can figure on the shell containing some 10-25 grams of explosive. (Which would roughly make sense givne the shells probably mass between 50-100 grams, or about the size/mass of 12.7-20mm shell in real life.)

by real life explosive, this comes out to between 40 and 100 kilojoule for TNT, up to 113-283 kilojoules for ONC. Given other eamples of explosives (basilisk shells, grenades, etc.) its quite likely that 40K explosives are at least as powerful as ONC, if not more powerful (many times more powerful than TNT, in other words.) Its quite possible 40K explosives are 5 or even 10x more powerful (especialyl going by grenade and basilisk references), which the "effects based analysis" above would concur with, as the increase would yield outputs more consistent with a grenade (hundreds of kilojoules)

A heavy bolter, ,incidentally, IIRC, is roughly twice the size/mass of a regular bolter shell, so it probably also has at least twice the explosive effect.

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Post by JBG » 2008-05-01 04:01am

Connor MacLeod, an interesting post. I haven't gamed 40K for years and certainly not under the present rules. Under the old rules I had about 5 to 6 thousand points worth of Space Wolves ( including Sydney's last old pattern Land Raider available for sale ). Also sitting in the games tallboy, though not mine, are Tyrranids, Eldar and Space Orcs, with some unpainted marines of a chapter I can't recall ( it is almost ten years - an original game chapter in green? ) and inquisitors etc. Even more Warhammer armies.

I always found the bolter to be an enigmatic weapon. My puppies used to fight the Eldar, Tyrranids and vast Orc armies but seemed best suited to slaughtering Orcs. The narrative behind all those early books referred to some sort of "centre of mass" sensor that enabled the bolt gun shell, against the"designed for" target, to penetrate and only explode once INSIDE the target. The weapon stats did not seem to reflect that crucial consideration so I just tooled up on Wolf Guard with special weapons or Wolf Guard Terminators ( I ended up with 18 and wanting more! ), as you could then with 40K and especially with Space Wolves.

Keep up the fruitful research.

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Post by Sea Skimmer » 2008-05-01 06:42pm

Different explosives release energy in different ways, some generate more heat, others more gases, detonation velocities are different and briasance (shattering effect, the speed at which pressure builds up basically) varies even more then anything else. Basically no single factor can provide a realistic comparison between explosives, the formula behind ‘Relative effectiveness factor’ tries to do that but it is of questionable value. The reality is that different explosives are good for different things, which is why everyone doesn’t just use the same explosive for everything (even ignoring the issue if mixed in stabilizers). So basically, don’t spend too much time trying to do math on this, unless you want to start churning out some spreadsheets
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Post by Swindle1984 » 2008-05-02 12:35am

My acquaintance with 40k is limited, so forgive me if this is a stupid question:

I've always heard bolters described as operating like gyrojets. That is, the propellant is in the projectile itself. If this is the case, then why does 40k art always show bolters ejecting brass like a regular firearm?
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Post by weemadando » 2008-05-02 12:42am

From the older novels (probably over-written in canon by now) what happened was the shell was fired conventionally, then once fired the gyrojet ignited.

Effectively it was a way of maintaining high velocity on the rounds and increasing range and accuracy.

After all, they aren't meant for spray-and-pray firing. To waste a round is an insult to the Emperor.

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Post by Academia Nut » 2008-05-02 12:44am

The bolter shell casings in the artwork is often considered to be just part of Imperial propoganda because bolter shells look awesome. For an example of this at work, look at the Ciaphas Cain artwork. None of the pictures on the front cover ever actually happen, especially since Cain doesn't use a bolter despite what the art would have you believe.

Also, the bolters are generally considered to have an initial propellant that gets the bolt out of the barrel at a considerable but subsonic speed before the gyrojet kicks in, which compensates for the problem of gyrojets having poor initial velocities.
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Post by Swindle1984 » 2008-05-03 03:32pm

Ah. So it works like an RPG, where the recoilless launch pops the round down range, then the rocket booster kicks in to accelerate it toward the target. Makes sense.

How are the rounds fused? Do they explode when the gyrojet propellant burns out, or only on impact?

Recoil can't be too heavy on a bolter. Based on descriptions, the shells are about the size of a 12-gauge shotgun shell, and gyrojets are infamous for having almost no recoil due to the method of propulsion. If this is the case, then why is that only the huge, armored Space Marines carry bolters on a regular basis? I've heard of Imperial Guard officers and commissars occasionally carrying them as sort of a reward for loyal service, but only small ones.

Out-of-universe, I can understand game balance issues, but what's the reason in-universe?
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Post by [R_H] » 2008-05-03 04:52pm

Swindle1984 wrote: Recoil can't be too heavy on a bolter. Based on descriptions, the shells are about the size of a 12-gauge shotgun shell, and gyrojets are infamous for having almost no recoil due to the method of propulsion. If this is the case, then why is that only the huge, armored Space Marines carry bolters on a regular basis? I've heard of Imperial Guard officers and commissars occasionally carrying them as sort of a reward for loyal service, but only small ones.

Out-of-universe, I can understand game balance issues, but what's the reason in-universe?
I thought bolt shells varied in bore and size, that for example the bolt pistols SMs carried fired different ammunition (and were about as large as a rifle for a non-SM...Eisenhorn) than those carried by a non-SM. What I do know is that heavy bolters are crew-served in the majority of Guard units, while SMs use them like an LMG or a SAW, but as far as I know, there is no mention if an IG heavy bolter is chambered for the same ammunition as an SM heavy bolter

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Post by Hawkwings » 2008-05-03 05:10pm

Swindle1984 wrote:Recoil can't be too heavy on a bolter. Based on descriptions, the shells are about the size of a 12-gauge shotgun shell, and gyrojets are infamous for having almost no recoil due to the method of propulsion. If this is the case, then why is that only the huge, armored Space Marines carry bolters on a regular basis? I've heard of Imperial Guard officers and commissars occasionally carrying them as sort of a reward for loyal service, but only small ones.

Out-of-universe, I can understand game balance issues, but what's the reason in-universe?
Space Marine bolters seem to have a stronger initial charge, enough that the bolt will penetrate and kill even at close range. They're tougher, they can handle it.

And Guard bolters are definitely different than SM bolters. Probably less powerful ammo and smaller propellant charges.
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Post by Connor MacLeod » 2008-05-04 04:34am

Swindle1984 wrote:Ah. So it works like an RPG, where the recoilless launch pops the round down range, then the rocket booster kicks in to accelerate it toward the target. Makes sense.
Its more like a hybrid of a slugthrower/gyrojet/grenade/missile launcher type weapon with the versatility of a shotgun.

Alot of normal weapons can apparently be "rocket propelled" - from Executioner rounds for shotguns and guided rounds for most slughtrowers (even pistols.. the more sophisticated of those can actually come about and hit a target multipel times before running out of propellant) - larger cannons (Earthshakers and battle cannon) can also use Rocket-assisted propulsion.
How are the rounds fused? Do they explode when the gyrojet propellant burns out, or only on impact?
Mass reactive warheads. THey're designed to detonate in the presence of mass (kind of like a more complicated impact fuse, I believe.)
Recoil can't be too heavy on a bolter. Based on descriptions, the shells are about the size of a 12-gauge shotgun shell, and gyrojets are infamous for having almost no recoil due to the method of propulsion. If this is the case, then why is that only the huge, armored Space Marines carry bolters on a regular basis? I've heard of Imperial Guard officers and commissars occasionally carrying them as sort of a reward for loyal service, but only small ones.
Maintenance is one reason, but another is the ammo is bulky and hard to produce. They carry these in magaiznes after all.

The main reason I figure why they might use both propellant and a rocket booster is because it allows for extreme speeds without high recoil. Its kind of a workaround for railguns and whatnot.

As an example: A non-saboted tank munition *might* get around 1000-1500 m/s muzzle velocity from a tank gun (A bit higher if they use some sort of enhanced tech like electrothermal-chemcial) or maybe a bit more if they have really good propellant, but recoil forces and momentum imparted to the tank really limit this (especially for more mobile tanks like the Conqueror). With rocket-assisted propulsion you can boost that higher (2+ km/sec ). OF course, rocketry can have some efficiency issues (most reaction drives will in terms of energy expended vs velocity gained, so there is a balancing act.

You can kind of think of a bolter in similar terms - the hybrid nature makes it very powerful, but its also more sophisticated compared to a simple slughthrower.

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Post by Connor MacLeod » 2008-05-04 04:38am

weemadando wrote:From the older novels (probably over-written in canon by now) what happened was the shell was fired conventionally, then once fired the gyrojet ignited.

Effectively it was a way of maintaining high velocity on the rounds and increasing range and accuracy.
Probably for most of the "normal" human bolters like officer's bolt pistols. Alot of sourcecs indicate bolters have recoil, especially space marine ones. (Hell the Munitorum manual I just covered indicated that. So does Grey Knights, Iron Hands, ,and a few others.)
After all, they aren't meant for spray-and-pray firing. To waste a round is an insult to the Emperor.
As of 3rd edition they were... they had a "burst mode" (3-4 shots per trigger pull) in the 3rd edition rule book. A number of novels had bolters with full auto settings.

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Post by weemadando » 2008-05-04 04:46am

Connor MacLeod wrote:
weemadando wrote:From the older novels (probably over-written in canon by now) what happened was the shell was fired conventionally, then once fired the gyrojet ignited.

Effectively it was a way of maintaining high velocity on the rounds and increasing range and accuracy.
Probably for most of the "normal" human bolters like officer's bolt pistols. Alot of sourcecs indicate bolters have recoil, especially space marine ones. (Hell the Munitorum manual I just covered indicated that. So does Grey Knights, Iron Hands, ,and a few others.)
Actually no, they were all described that way.

RAAARK-pop-whooooosh-THUD-KRUMP according to Ian Watson's books.

Raaark being the bolter actually firing.
Pop being the gyrojet igniting.
Whoosh being it jetting towards target
Thud of it hitting the thing
Krump of it detonating inside the target.

That's for an inquisitors bolt pistol, a stunties bolter, space marine bolters - all are described in that same way.

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Post by Connor MacLeod » 2008-05-04 04:49am

Oh: And insofar as "bolt" sizes go, they all pretty much seem to be roughly .75 calibre or so - ora t least there are cases of both "normal" human bolters and Space MArine versions both using .75 calibre shells. I am sure they can make smaller (they make larger for heavy bolters) but the main difference I would imagine comes in the length of the shell rather than diameter (it may also be a density issue - Spacee Marine tech tends to bea lot more sophisticated, so their explosives and propellents could be denser, for example.)

Longer shells also probably differentiate between bolt pistols and bolt guns as well (And also help explain why the latter can be so bulky, Space Marine or normal human versions both.)

And the whole "casing" thing isn't just "artwork" neccecsarily, a number of novels (The Necromunda and the Soul drinkers novels) have depicted bolters ad ejecting casings. Hell, the cutaway in the 3rd edition rulebook depicts "ejection ports" on a storm bolter, and they depict the rounds as having casings too. On the other hand, some novels (like the Space Wolves) say that bolters can use "caseless" ammunition. So it probably depends on model and who is making it. (I'd also point out some autoguns/stubbers use caseless ammo too.)

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Post by Connor MacLeod » 2008-05-04 05:01am

weemadando wrote:Actually no, they were all described that way.

RAAARK-pop-whooooosh-THUD-KRUMP according to Ian Watson's books.

Raaark being the bolter actually firing.
Pop being the gyrojet igniting.
Whoosh being it jetting towards target
Thud of it hitting the thing
Krump of it detonating inside the target.

That's for an inquisitors bolt pistol, a stunties bolter, space marine bolters - all are described in that same way.
Not really. "Space Wolf" By william king depicted Space Wolf bolters as operating differently (The caseless bit, and Ragnar explicitly describes them still having recoil.) Draco's novels depicted them as being far noisier, as well as having little to no recoil

For that matter, Abnett's "first and Only" novel isn't really THAT much older than "Draco" (the first novel of the Inquisition War) either. Or Nightbringer. That's not even getting into all the short storie compilations or Execution Hour (I havent read EH in awhile, but I can recall them being differently than depicted in the IW novels.)

At best, you can just say "Ian Watson wrote them all that way" -most authors tend to differ in their minds how the weapons work to some degree or another. (IE Ben Counter's tendency to treat plasma weapons like some sort of super-soaker flamethrower.)

Edit: Actually I was wrong. Inquisiton war (Harlequin) was first published in '95, s that was older.
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Post by weemadando » 2008-05-04 05:06am

I've always liked Watson's description as it's reasonable and matches up pretty well with the various descriptions - conventionally fired (caseless or not), gyrojet involvement and variable fusing for the warhead.

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Post by Connor MacLeod » 2008-05-04 05:28am

weemadando wrote:I've always liked Watson's description as it's reasonable and matches up pretty well with the various descriptions - conventionally fired (caseless or not), gyrojet involvement and variable fusing for the warhead.
Actually I thought it was pretty cringe worthy. The bolt is kicking in no more than a couple of feet away (less for the squat) and you're talking about very hot, very HIGH velocity exhaust, even if you assume the majority of the shell's mass is propellant. It would be upleasant at least, quite possibly dangerous at that proximity.

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Post by NecronLord » 2008-05-04 12:36pm

Connor MacLeod wrote:Actually I thought it was pretty cringe worthy. The bolt is kicking in no more than a couple of feet away (less for the squat) and you're talking about very hot, very HIGH velocity exhaust, even if you assume the majority of the shell's mass is propellant. It would be upleasant at least, quite possibly dangerous at that proximity.
That's a problem with all bolts. Though personally, I never got the impression that the propellant was that dangerous. AFAIK, real gyrojets didn't/don't scorch the user.

As for their being noisy - as a rule, they're a weapon designed to inspire or terrorise. Noisy is good. Most bolts might well be designed to be as noisy as possible.
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Post by Swindle1984 » 2008-05-04 02:27pm

I'm gonna clear up a couple issues about gyrojets, just because so few people have any experience with them.

Real gyrojets don't present any more of a threat of scorching or otherwise harming the user than the muzzle flash of a standard firearm. And considering that, rather than using rifling, early gyrojets used ports in the barrel for the exhaust to go through in order to give the bullet spin, that's saying something.

Recoil in gyrojets was nearly non-existent. They were also extremely quiet; they sounded about the same as someone opening a can of soda. A friend of mine used to fire his gyrojet pistol in camp and no one would even glance over at the noise. He stopped shooting it when ammo became almost non-existent, so he's never let me shoot it. The pistol itself isn't worth much, but the little ammunition he has left is worth about $50 a round now because it's so scarce.

Gyrojets did not, as the story goes, have such low muzzle velocity that if you put a bottle in front of the barrel it would trap the bullet. It had sufficient muzzle velocity to punch through a bottle, plastic or glass, and keep going. Somehow people confused the fact that it continues accelerating after leaving the barrel with "it isn't lethal at point-blank range".

Gyrojets were wildly inaccurate at distances beyond 25 meters because of the weight of the bullet changing as the propellant burned. Imperium bolters using cases as the initial propulsion solves this problem as well as the "low" muzzle velocity issue.




As for the length of shells being a way to differentiate between Space Marine bolters and Guard bolters, that may be the case. Standard 12-gauge shotgun shells (about .78-caliber) are 2 3/4" in length. Magnum shells are 3" in length. The Mag-7 shotgun, in order for the magazine to fit into the pistol grip without it being huge, use 12-gauge shells that are only 40mm in length. Aguila, a Mexican ammunition manufacturer, makes "mini shells" in 12-gauge that are even shorter, allowing one to carry more ammunition in a 12-gauge shotgun. I don't know how much performance is affected in the mini shells.

So it is entirely possible that all bolters are the same caliber (which would save on logistics for things like barrels, bolt faces, etc.) but different models use shells of different lengths.
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Post by Cykeisme » 2008-05-04 06:55pm

Swindle1984 wrote:Ah. So it works like an RPG, where the recoilless launch pops the round down range, then the rocket booster kicks in to accelerate it toward the target. Makes sense.
This can't be the case. No boltgun models that we've seen have a backblast.

If the back of the tube is closed (as in a boltgun), there is little effective difference between having a slugthrower-style charge and simply firing the rocket motor from the get-go.
You'll also have full recoil.

Of course, the combustion and chamber pressure curve of cordite differs form what one may use as rocket propellant, but since boltguns are designed to do what they do (we're analyzing an already working device, not designing one), we can assume they got around that.
It's possible the rocket may be a two-stage or something (simply by having two layers in the chunk of solid fuel perhaps.
Swindle1984 wrote:Real gyrojets don't present any more of a threat of scorching or otherwise harming the user than the muzzle flash of a standard firearm.
I'm sure the exhaust gases from those little gyrojet projectiles in real life don't pose any threat to the firer, but a boltgun shell is considerably heavier which would require a commensurate increase in velocity and/or mass of exhaust ejecta.

Since Space Marines tend to be wearing power armor and are big tough guys (to understate), I'm sure they can take it.
This may also explain the shield that we invariably see mounted on Imperial Guard heavy bolters; it may do more than protect the gunner from enemy fire.
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Post by Swindle1984 » 2008-05-04 10:33pm

Cykeisme wrote:
Swindle1984 wrote:Ah. So it works like an RPG, where the recoilless launch pops the round down range, then the rocket booster kicks in to accelerate it toward the target. Makes sense.
This can't be the case. No boltgun models that we've seen have a backblast.
*SIGH* I wasn't being literal. :roll: It was an example of similarities.

An RPG uses an initial propulsion to get the projectile a short distance down-range, then the rocket kicks in and accelerates it. A bolter uses an initial propulsion to get the projectile a short distance down-range, then the rocket kicks in and accelerates it.

Comprende?
If the back of the tube is closed (as in a boltgun), there is little effective difference between having a slugthrower-style charge and simply firing the rocket motor from the get-go.
You'll also have full recoil.
I wasn't saying that bolters are fucking recoilless guns. That much is REALLY obvious just looking at one.

Firing the rocket means the projectile has to take time to accelerate. Firing it like a conventional firearm and having the rocket booster kick in afterwards gives it an already high velocity right out the muzzle before it begins accelerating from the rocket. Higher velocity means greater penetration.
Of course, the combustion and chamber pressure curve of cordite differs form what one may use as rocket propellant, but since boltguns are designed to do what they do (we're analyzing an already working device, not designing one), we can assume they got around that.
It's possible the rocket may be a two-stage or something (simply by having two layers in the chunk of solid fuel perhaps.
Except that from artwork and descriptions we know that bolters DO fire like conventional firearms before the gyrojet projectile ignites. Did you read the thread?

quote]
Swindle1984 wrote:Real gyrojets don't present any more of a threat of scorching or otherwise harming the user than the muzzle flash of a standard firearm.
I'm sure the exhaust gases from those little gyrojet projectiles in real life don't pose any threat to the firer, but a boltgun shell is considerably heavier which would require a commensurate increase in velocity and/or mass of exhaust ejecta.[/quote]

A real-life gyrojet was 13mm for the original and 12mm (.50-caliber, the largest caliber a firearm in the US can fire without being classified as a destructive-device). A bolter is .75-caliber, which is about the size of a 12-gauge shotgun shell, slightly smaller. The difference in size isn't all that important.

Having dabbled in model rocketry, the exhaust in something that size wouldn't be an issue, especially since the initial firearm-style propulsion would have gotten it at least several yards, possibly several dozen years, down-range before the rocket booster kicked in. And since we know Guardsmen carry hand-held bolters, the exhaust can't be a problem for them.
Since Space Marines tend to be wearing power armor and are big tough guys (to understate), I'm sure they can take it.
This may also explain the shield that we invariably see mounted on Imperial Guard heavy bolters; it may do more than protect the gunner from enemy fire.
The shield is likely aesthetic (much like some Guardsmen look like WWI German soldiers) rather than functional, designed to make the heavy bolter look like WWI and older machine guns, which often came on carriages and had similar shields installed.

Image

That shield was useless for protecting the gunners from incoming fire, and the muzzleflash of the gun wasn't much of an issue to say the least, so the purpose of it being there is lost on me.
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Post by Cykeisme » 2008-05-05 07:05pm

Swindle1984 wrote:
Cykeisme wrote:
Swindle1984 wrote:Ah. So it works like an RPG, where the recoilless launch pops the round down range, then the rocket booster kicks in to accelerate it toward the target. Makes sense.
This can't be the case. No boltgun models that we've seen have a backblast.

*SIGH* I wasn't being literal. :roll: It was an example of similarities.

An RPG uses an initial propulsion to get the projectile a short distance down-range, then the rocket kicks in and accelerates it. A bolter uses an initial propulsion to get the projectile a short distance down-range, then the rocket kicks in and accelerates it.

Comprende?
I was pointing out your error.

Lets look at this phrase: "it works like an RPG, where the recoilless launch pops the round down range".

If you "pop the round down range", you don't have a recoilless launch. How the hell do you expect it to be recoilless if you're popping the round down range? Forgetting conservation of momentum?

The reason why simple RPGs have no recoil when they're launched is because the tube is vented at the back; along the axis of the launch, there is no force transferred to the launcher. The rocket takes off purely due to the reaction of the exhaust gases leaving the rear of the tube. Hence the backblast.

The only way around the backblast is to have non-dangerous countermass ejected out the rear of the tube, but that's out of the question, since you were "making an example of similarities", and boltguns have no countershot ejection either.

If you have a charge to "pop the round downrange", you'll have recoil.

No recoil? Backblast.
No backblast? Recoil.

Your example is erroneous.

Swindle1984 wrote:
If the back of the tube is closed (as in a boltgun), there is little effective difference between having a slugthrower-style charge and simply firing the rocket motor from the get-go.
You'll also have full recoil.

I wasn't saying that bolters are fucking recoilless guns. That much is REALLY obvious just looking at one.
Oh, you weren't saying that? My mistake.
It's just that you started out your post by comparing boltguns to recoilless RPGs (never mind your example was erroneous).

Swindle1984 wrote:]Firing the rocket means the projectile has to take time to accelerate. Firing it like a conventional firearm and having the rocket booster kick in afterwards gives it an already high velocity right out the muzzle before it begins accelerating from the rocket. Higher velocity means greater penetration.
No argument there.

Swindle1984 wrote:
Cykeisme wrote:Of course, the combustion and chamber pressure curve of cordite differs form what one may use as rocket propellant, but since boltguns are designed to do what they do (we're analyzing an already working device, not designing one), we can assume they got around that.
It's possible the rocket may be a two-stage or something (simply by having two layers in the chunk of solid fuel perhaps.

Except that from artwork and descriptions we know that bolters DO fire like conventional firearms before the gyrojet projectile ignites. Did you read the thread?
Did you read what I said? You even quoted it elsewhere. Here:
Cykeisme wrote:If the back of the tube is closed (as in a boltgun), there is little effective difference between having a slugthrower-style charge and simply firing the rocket motor from the get-go.
I further supposed that the expansion curve of rocket fuel may not be optimal, hence beginning the burn phase with a different type of propellant, which combusts during the time the shell travels within the barrel.

Did you notice that a boltgun shell has a baseplate made of metal, with just an exhaust nozzle opening? If there's no casing left behind to be ejected, that means that same baseplate we see is part of the projectile itself. Nothing is left behind: no casing, no propellant
Thus a bolter CANNOT fire like a "conventional firearm". It cannot possibly even fire like a caseless firearm. The entire shell travels down the barrel, leaving nothing behind.
It doesn't matter if there's a first-stage charge with a gas pressure curve similar to firearm propellant for the in-barrel travel; if all the propellant is in the shell as it travels, releasing expanding gas through the exhaust port at its rear, it's a rocket right from the start.

You will still get the same effect:
- a high muzzle velocity due to the pressure buildup within the enclose chamber and barrel
- a bang as that built-up pressure explodes out of the barrel after the bolt clears the muzzle
Yet, even disregarding the after-muzzle rocket propulsion, it is not like a conventional firearm.

Swindle1984 wrote:
Cykeisme wrote:
Swindle1984 wrote:Real gyrojets don't present any more of a threat of scorching or otherwise harming the user than the muzzle flash of a standard firearm.
I'm sure the exhaust gases from those little gyrojet projectiles in real life don't pose any threat to the firer, but a boltgun shell is considerably heavier which would require a commensurate increase in velocity and/or mass of exhaust ejecta.

A real-life gyrojet was 13mm for the original and 12mm (.50-caliber, the largest caliber a firearm in the US can fire without being classified as a destructive-device). A bolter is .75-caliber, which is about the size of a 12-gauge shotgun shell, slightly smaller. The difference in size isn't all that important.

Having dabbled in model rocketry, the exhaust in something that size wouldn't be an issue, especially since the initial firearm-style propulsion would have gotten it at least several yards, possibly several dozen years, down-range before the rocket booster kicked in. And since we know Guardsmen carry hand-held bolters, the exhaust can't be a problem for them.
I'm not even saying that bolter exhaust should be a hazard to gunners, the constant gyrojet comparisons were just getting on my nerves. For fucks sake, we aren't talking about model rockets or little gyrojet pistols.
The difference in size isn't all that important? Aside from the fact that a 50% increase in linear dimensions comes up to over three times the volume, never mind the density of the projectiles involved.
Anyway, I see how you like to compare the size of the shells there without mentioning velocity or mass. Nice.

Since Space Marines tend to be wearing power armor and are big tough guys (to understate), I'm sure they can take it.
This may also explain the shield that we invariably see mounted on Imperial Guard heavy bolters; it may do more than protect the gunner from enemy fire.

The shield is likely aesthetic (much like some Guardsmen look like WWI German soldiers) rather than functional, designed to make the heavy bolter look like WWI and older machine guns, which often came on carriages and had similar shields installed.

IMAGE

That shield was useless for protecting the gunners from incoming fire, and the muzzleflash of the gun wasn't much of an issue to say the least, so the purpose of it being there is lost on me.
I agree, it's unlikely that boltgun shell exhaust is a firer hazard.

However, let's see here.. you begin with a non-SoD, out-of-universe supposition that the shield on a Guard heavy bolter is to make it look like a WWI machine gun.
Subsequently, you state that WWI MGs had useless shields on them.
Therefore, the shields on Guard heavy bolters are likely useless.
Got it.
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Post by weemadando » 2008-05-05 07:26pm

The bolter gyrojet DOES NOT ENGAGE until well after the bolt has left the barrel. As I mentioned previously - every version that I have read about points to the gyrojet engaging as a velocity sustaining measure.

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Post by Sea Skimmer » 2008-05-05 07:50pm

So in other words you folks shouldn’t be calling these damn things gyrojets at all, its just RAP, rocket assisted projectiles. Using accurate terminology avoids confusion.
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Post by weemadando » 2008-05-05 07:57pm

Sea Skimmer wrote:So in other words you folks shouldn’t be calling these damn things gyrojets at all, its just RAP, rocket assisted projectiles. Using accurate terminology avoids confusion.
Yeah, pretty much.

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