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Supposedly in addition to range, the U.S. Marines have kept the M16A4 as the primary long gun because of the 20-inch barrel being the intended barrel length; there's anecdotes of the 5.56x45mm round failing to fragment or yaw when fired out of the 14.5-inch M4A1 barrel but doing so more when fired from the 20-inch M16A4 due to the higher velocity.

adam_grif, for official "improved 5.56mm" variants there's the Mk 262 77 grain round (GlobalSecurity.org has more, and here's the manufacturer), and the Mk 318 Mod 0. (Link to the MC Times article here, and numbers on the claimed ballistics and design here.) There's also the new lead-free M855A1 "Enhanced Performance Round" variant of the regular M855; at least in line with "carbines as the norm," it's claimed to have been designed with the M4-type carbine (and the 14.5 inch barrel) in mind; both the Mk 318 and the M855A1 are 62 grain. (Accurate Shooter has a cutaway image of the M855A1 and a bar graph showing claimed improved barrier penetration, and here's the Corps taking a look at the round too.) No idea where the MC Times got a claimed 2,925 fps out of a 14.5 inch barrel though!

For comparison, Knight's Armament Company claims 2,425 fps (739 m/s) and 848 ft-lbs (1149.7184 J) for their 6x35mm round (65 grain) out of their PDW's 10-inch barrel (it also has an 8-inch barrel option), compared to a claimed 2,375 fps (723.9 m/s) and 776 ft-lbs (1052.1 J) for the 5.56x45mm (62 grain) out of a 10-inch barrel. (Disclaimer in that as of now, Wikipedia's KAC PDW article claims that other manufacturers have reported higher velocity/energy for the 5.56 than what KAC claimed.) EDIT: I agree, I'm not holdng my breath on KAC ever clarifying this, as interesting as it is.

[R_H], I actually think that the REC7's seeming obscurity has two reasons -- it's not a H&K, and I think that the REC7 nowadays looks "just" like any other "tacticool" M4-type; at least back when it was the M468, it had a visually distinct ARMS S.I.R.. In addition, H&K was very effective at getting their product out into the public eye I felt.

Capt_Hawkeye, the version I'd heard of .40 S&W's story was that it resulted from the FBI underloading the 10mm Auto after some agents had problems with controlling the 10mm Auto, until eventually "the underload" became a caliber of its own.

Mr. Coffee wrote:

Stark wrote:How does the mass and energy of a round vary with barrel length? What I mean is, 556 is a very light and fast round, and reducing the velocity obviously sucks, and heavier rounds are going to be less fussy - but do all rounds benefit from similar lengths? Are any rifle rounds going to be any good in a 12" barrel or do you need fat and slow (ie pistol) rounds?

Longer barrels allow for longer burn of the power which makes for higher pressures behind the bullet before it exits the barrel, hence higher velocity. At least that's how it was explained to me when I was a wee lad learning which end of the weapon points downrange.

What I meant was what attributes make a round effective in shorter barrels (without hilarious flash problems). I'm interested that the 6x35 performance, since I'd imagine it uses the same prollant and isnt that much thicker. Are the modern intermediate rounds longer in the case and thus heavier than you'd expect?

[R_H] wrote:Barrett REC7 (as it has a mid-length short stroke piston) rarely comes up.

Brand name. H&K is a lot older and better established making well known models of machineguns, rifles, and subguns. In contrast the only really big number of sales Barrett has are in the form of anti-material rifles. Being better established H&K also has more money to throw at advertising.

Stark wrote:What I meant was what attributes make a round effective in shorter barrels (without hilarious flash problems). I'm interested that the 6x35 performance, since I'd imagine it uses the same prollant and isnt that much thicker. Are the modern intermediate rounds longer in the case and thus heavier than you'd expect?

increasing the bullet's mass or using a faster burning powder (not more powder, just faster rate of burn) would be the first things that spring to mind. Hilarious flash isn't that big a problem, pretty easily solved with either a good flash suppressor or sound suppressor.

As far as 6x35mm goes, couldn't tell you. I've never handled it or fired it, but I will say that claims are nice and all, but until you see independently confirmed chronometer information on their claims I'd take it with a grain or two of salt.

How does having the tolerances for faster-burning propellant affect other aspects of the gun?

Stark wrote:How does having the tolerances for faster-burning propellant affect other aspects of the gun?

The faster the propellant burn rate is, the more propellant burns, and thus produces high pressure gases to propel the bullet in a given barrel length, i.e. the bullet accelerates faster. In a shorter barrel this makes for more velocity before the bullet reaches the the muzzle, or in AR-15 derrivitive, before it reaches the gas tube.

Not only that, it would probably also cause a really high pressure spike and KB the gun. If you where aiming to get the same kind of velocity out of a 5.56 round in 10" rather than the preffered 20" that is.

It would be better to accept as much of the velocity loss as you can and aim to reduce the amount of unburnt powder, which causes a lot of flash. Otherwise you would probably need a new design with thicker brass and beefier chamber walls to maintain the required higher pressures to push a bullet to those kinds of velocities in a short barrel, that would probably also cause a shorter barrel life. Hmm all these compromises are starting to remind me a lot of the 5.7x28mm now.

Going over the old PDR stats sheet put out by Magpul, the bar graph on page 3 claimed muzzle velocities of about ~3200 fps for the "M16," ~3000 fps for the "M4," and ~2800 fps for an 11.5 inch barrel; it also made the claim of 2700 fps needed for "max fragmentation," and 2500 fps for "partial fragmentation." Based on that, the corresponding fragmentation ranges were claimed to be ~350 feet, ~225 feet, and ~150 feet respectively... in a sign of how outdated the thing is, it claimed that the "common self defense" range was out to 140 feet, and the "common combat range" out to 200 feet. Hopefully someone more familiar with ballistics than me can do the math to show how quickly the claimed velocity (or speed?) loss was over distance.

For anyone wondering what the hell I'm talking about... think a P90-shaped bullpup rifle in 5.56x45mm using standard magazines. (The caliber and magazine choice were explicitly for "logistics" reasons.) The use of a SBR barrel in a bullpup may sound self-defeating, but the claimed dimensions are very similar to the P90, "small" enough that a man could actually wear it alongside his torso under his armpit. If it ever does see the light of day in actually-firing, non-mockup form, it'll be interesting to see whether Magpul can keep its 'promise' that it would support conversion to "6mm, 6.5, 6.8 etc." (I presume that to mean KAC's 6x35mm, plus 6.5mm Grendel and 6.8x43mm.)

Edward Yee wrote:Going over the old PDR stats sheet put out by Magpul, the bar graph on page 3 claimed muzzle velocities of about ~3200 fps for the "M16," ~3000 fps for the "M4," and ~2800 fps for an 11.5 inch barrel; it also made the claim of 2700 fps needed for "max fragmentation," and 2500 fps for "partial fragmentation." Based on that, the corresponding fragmentation ranges were claimed to be ~350 feet, ~225 feet, and ~150 feet respectively... in a sign of how outdated the thing is, it claimed that the "common self defense" range was out to 140 feet, and the "common combat range" out to 200 feet. Hopefully someone more familiar with ballistics than me can do the math to show how quickly the claimed velocity (or speed?) loss was over distance.

The 2700fps also comes up on Arfcom's Ammo Oracle page/pdf. That's what Fackler determined was the velocity needed for M193 and M855 to reliably fragment.

At 75F, 25% RH, sea level
Out of a 14.5" barrel: M193 fragments out to 95-100m
M855 fragments out to 45-50m

Out of a 20" barre:l M193 fragments out to 190-200m
M855 fragments out to 140-150m

As to mitigating flash, there's also flash retardant.

This kind of stuff is why pistol-caliber ammo designed to use in pistols is not the same (generally weaker) than the same size round intended for use in a submachinegun. A 9mm pistol round for, say, a Glock is not going to be as effective fired from a MP5. And an MP5 9mm round will be a "hot" load for a pistol, possibly risking damage to the pistol.

A .45 submachinegun would be effective, hot-loaded and intended for very close range. Hence the Thompson's generally positive reputation. But I'd be willing to bet that extended use of Thompsons in a long range, open-field environment would reveal deficiencies.

In an ideal world, we could issue weapons with appropriate ammo to soldiers based on the terrain they were going to fight in-- L1A1s, maybe, for wide open fields, then M-16s or AK-47s, perhaps, for rolling hills and forests, then carbines and "long barrel SMGs" (ie, Thompsons, as opposed to micros like the Mac-10) for urban work. Sigh.... a perfect world.

Carbine length weapons are probably the best compromise someone could think of, considering the fighting conditions face din Iraq and A-stan. Bear in mind other things like vehicle-dismount-ability is being factored in, as well as other "non-firing" parameters. I don't know if I'd make the same choice, but then, I do appreciate the constraints being worked under. I just think a re-evaluation of the M-16 family has been needed for some time; recent events just throw this into the spotlight-- but that's all my opinion.

If it had a real folding stock the dismountability of the AR-15 series would certainly be a hell of a lot better.

Shame about that rear buffer I guess.

Funny thing is, there's several workarounds using a M4-reminiscent shape, mainly by putting the recoil/return spring/system above the barreland/or operating rod, from the old OA-93 to Para Ordnance's TTR ("Tactical Target Rifle") and Rock River Arms' PDS ("Piston Driven System") carbine and pistol... even Colt got into this! They apparently merely shortened the tube, but enough so that it would allow a collapsible/telescoping and side-folding (oddly enough, to the left, "inwards" for a right-handed user) stock, with fold-down butt plate; so far this has only been exhibited on their "Sub-Compact Weapon" SBR with 10.3 inch barrel. Also, at DSEi '09 H&K showed a HK416 variant with 9.3 inch barrel and, apparently through an also-shortened tube, a G3/MP5-style stock.

Speaking of that rear buffer though, supposedly the utility of a fixed stock for hand-to-hand is part of the reason why the Marine Corps is still loath to even allow a Canadian C7-style variant (20 inch barrel with telescoping stock), much less use the M4A1 more often.

P.S. Chen Lee's Shot Show 2010 photo collection for the Colt Tactical line has more SCW photos (you can find them on several forums), but the HK416 link has the only photos I know of for the DSEi '09 variant.

It's still longer and more fiddly than on a real side folder.

Also the colt SCW is HIDEOUSLY ugly

Edward Yee wrote:Also, at DSEi '09 H&K showed a HK416 variant with 9.3 inch barrel and, apparently through an also-shortened tube, a G3/MP5-style stock.

There's manufacturers that go even shorter then that. Noveske have the "Diplomat" 7.5" barrel varient of their N4 in both 5.56 and 6.8. Not sure if they shortened the gas tube or not. I'd love to see chronos of just how much velocity 5.56 loses when fired from that short a barrel.

JointStrikeFighter wrote:It's still longer and more fiddly than on a real side folder.

If you mean the SCW stock, absolutely, but it shows that (for better or for worse) even Colt will "cave" on this point... not sure if it's compatible with any of their other products though, or whether the shorter buffer tube (that allows this) will only work with a SBR.

Mr. Coffee wrote:

Edward Yee wrote:Also, at DSEi '09 H&K showed a HK416 variant with 9.3 inch barrel and, apparently through an also-shortened tube, a G3/MP5-style stock.

There's manufacturers that go even shorter then that. Noveske have the "Diplomat" 7.5" barrel varient of their N4 in both 5.56 and 6.8. Not sure if they shortened the gas tube or not. I'd love to see chronos of just how much velocity 5.56 loses when fired from that short a barrel.

They claim a 1:7 twist (polygonal rifling) and 1/2x28 threads for their 5.56 barrels, or 1:12 twist (also polygonal rifling) for the N6's barrels (with 5/8x24 threads) in 6.8x43, if that would make any difference. For the N6's barrels for 7.672x51 they claim to also use 5/8x24 threads but a 1:10 twist instead.

Interestingly, Primary Weapons Systems, Inc. (PWS) offers a "MK107" piston upper ("Diablo") with 7-inch barrel in 5.56 or... of all things, 7.62x39. For the 5.56 version they claim 2300 fps and 645 ft-lbs at the muzzle, or 2699 fps and 889 ft-lbs for their 10.5 inch barrel upper/rifle ("MK110"), both using "XM193" ammo. The 7.62x39 version of their "MK107" has a claimed 1960 fps and 1040 ft-lbs using Wolf 122 grain FMJ. Both MK107 versions use stainless steel barrels with have polygonal bore (rifling?) and a 1:8 twist, while the MK110 has "button" rifling and a chrome-lined barrel with 1:8 twist... if that'd make a difference in the ballistics, much less reliability.

LWRC offers the M6A2 with a 16.1 inch, 14.7 inch, 12.7 inch, or 10.5 inch barrel, plus the PSD with 8 inch barrel; both are claimed to support 6.8x43, but LWRC posted no velocity or energy claims.

According to this separate and unsourced post, the special HK416 was to meet a Brit requirement, "entirely a developmental project and only a few are being hand fitted for the customer." Here's the only post I know of involving anyone actually handling it, albeit not fired.

Edward Yee wrote:They claim a 1:7 twist (polygonal rifling) and 1/2x28 threads for their 5.56 barrels, or 1:12 twist (also polygonal rifling) for the N6's barrels (with 5/8x24 threads) in 6.8x43, if that would make any difference. For the N6's barrels for 7.672x51 they claim to also use 5/8x24 threads but a 1:10 twist instead.

Noveske sells the Diplomat in 7.62x51? Hilarious. There's a reason why KAC went with a 16" barrel for the SR-25EMC, and the same for LMT and the MWS.

Edward Yee wrote:LWRC offers the M6A2 with a 16.1 inch, 14.7 inch, 12.7 inch, or 10.5 inch barrel, plus the PSD with 8 inch barrel; both are claimed to support 6.8x43, but LWRC posted no velocity or energy claims.

Stephen H owns a PSD in 6.8, maybe he's posted velocities. Either him or DocGKR.

[R_H] wrote:Noveske sells the Diplomat in 7.62x51? Hilarious. There's a reason why KAC went with a 16" barrel for the SR-25EMC, and the same for LMT and the MWS.

The N6's don't have a Diplomat configuration (Those are all N4s in either 5.56 or 6.3). Shortest barrel they offer for the 7.62x51mm N6 is 12.5".

Mr. Coffee wrote:
The N6's don't have a Diplomat configuration (Those are all N4s in either 5.56 or 6.3). Shortest barrel they offer for the 7.62x51mm N6 is 12.5".

Ah. There's still gonna be some hellacious muzzle blast with that short a barrel. The ballistics wouldn't be too great either, but on the upside, I bet the flash could singe the target's eyebrows off.

If america wasn't terrified of bullpups this would probably be a moot point.

[R_H] wrote:

Mr. Coffee wrote:
The N6's don't have a Diplomat configuration (Those are all N4s in either 5.56 or 6.3). Shortest barrel they offer for the 7.62x51mm N6 is 12.5".

Ah. There's still gonna be some hellacious muzzle blast with that short a barrel. The ballistics wouldn't be too great either, but on the upside, I bet the flash could singe the target's eyebrows off.

Actually,the muzzle blast doesn't look that bad. Lot of smoke, but not really that much more than you'd expect from a bog standard M4 firing off 20 rounds on full auto. Probably has something to do with Noveske's fuck-off huge KX3 flash supressor.

JointStrikeFighter wrote:If america wasn't terrified of bullpups this would probably be a moot point.

The Magpul PDR proposal sheet had a "forward" magazine release in the general area where the AR-15 magazine release would be, no idea how that would have been made to work, but the "less transition to make" (training-wise) combined with it being "oh, no magazine release in the back" rationale was obvious.

The 5.56 and 6.8 models from Noveske are called N4, the 7.62x51 stuff are the N6 line. Shoulda made it more clear myself that the 7.62x51's shortest was the 12.5 inch "Leonidas" (ahem).

EDIT: If you're not a fan of the SBR craze/"fad"/trend... there was worse: The "M16X" (4 inch barrel) existed (supposedly built by M2 Corp/Military Manufacturing Corporation), and was supposedly tested by Canadian Forces, along with an "M16C" (6 inch barrel) and "M16SP" (8.375 inch barrel), for the PDW role, along with a 5 inch barrel SBR and other barrel lengths, as mentioned here. (He may or may not be this guy.) Egads much? Ferfrans claims to offer their own 6 and 8 inch barrel PDWs, and there's video here... links ahoy, I know, but proof that such... things exist(ed).

I remember reading about how various armies had a not-dissimilar problem in the run-up to the Great War, trying to get a 20-inch cavalry carbine and a 30-inch rifle to fire the same cartridge satisfactorily.
The eventual solution? Ditch separate infantry and cavalry weapons in favour of an intermediate-length weapon for both services.
A bullpup weapon would obviously make this easier without losing performance, and I'm pretty sure that's what the British Army do with the SA80; a carbine version does exist, but it's only issued to AFV and gunship crews in case they have to bail from their vehicle. If the US Army really is committed to a conventional layout, however, why not an intermediate-length barrel -16.5 inches, maybe- combined with the telescoping stock from the M4?

Zaune wrote:the SA80; a carbine version does exist, but it's

probably the ugliest weapon ever made.

Also, the layout makes it so short that its actually possible to have your hand on the pistol grip and put your trigger finger over the end of the muzzle. (Not that this is a design flaw, it just looks comical)

16-inch "M4" variants already exist due to a minimum-16-inch barrel requirement for 'regular' rifles (without having a Class III license or something?) -- for example, why you'll see both FN's P90 and F2000 with longer-barreled "sporting" variants -- but I'm not aware of any ballistics numbers for that barrel length.