It's interesting when you go back and look at the original 1984 Terminator Gun store scene.I've always been of the opinion that organized military force would allow for pretty rapid destruction of 800-series Terminators sent back into the past (one antitank rocket through the back and bye-bye endoskeleton), so Sarah Connor getting the drop on 800-series units as they chronoport seems entirely reasonable.
Maybe she was just late to the party this time, and the job usually consists of laying a few command-detonated antitank mines under the expected chronoport location (or not, GPS isn't quite accurate enough for that unless Carl has DGPS).
Lots of people over the years have gone over that scene:
Why would the Terminator ask for a weapon that surely only exists in the distant future?The Terminator : The 12-gauge auto-loader.
Pawn Shop Clerk : That's Italian. You can go pump or auto. [hands the Terminator the pump action shotgun]
The Terminator : The .45 long slide, with laser sighting.
Pawn Shop Clerk : [hands the Terminator a .45 gun from the glass case in front of the clerk ] These are brand new; we just got them in. That's a good gun. Just touch the trigger, the beam comes on and you put the red dot where you want the bullet to go. You can't miss. Anything else?
The Terminator : Phased plasma rifle in the 40-watt range.
Pawn Shop Clerk : [annoyed] Hey, just what you see, pal!
The Terminator : [looks around] The Uzi nine millimeter.
Why would T800 ask for a futuristic weapon
Basically, over the years the basic idea has been that this scene was to:
A.) Show that Terminators are not the cleverest things; being predisposed to direct action.
B.) Show fluff to fill in backstory: Silberman calls them ray guns; we get a small piece of the future this way.
But others have pointed out that it ties into the early mythology of the Terminator series regarding records destruction:
"Many records were lost, it was just being methodical. Skynet only knew the city and time that Sarah Connor was in."
Skynet's own records of 1984 military technology may not be the most accurate -- it knows that Westinghouse was researching them, but it may not know when exactly they became widespread military (and eventually civilian) issue.
When the shop owner says "just what you see," it passes the following information to the T-800:
1.) Plasma weapons don't yet exist on a wide scale. If they existed, the shop owner would have likely said: "Forty watt? The only thing they've got is that one watt thing by Westinghouse that weighs 500 pounds." since gun rags would have talked about it; and the implications of somewhat (vaguely) man portable quasi-energy weapons.
2.) Feeding from 1, it now has confirmed the threat level of the shop owner and organized forces (police/military) to be largely ballistic weapons only. Thus, it can reasonably begin active measures towards it's mission objectives -- measures that have a high risk of blowing infiltration. So it is now confident in killing the shop owner, and shooting people down in broad daylight.
As someone on reddit pointed out:
Imagine if the shop owner was a sarcastic asshole and said "we're all out of those, someone bought the last one." The T-800, not understanding sarcasm, would have been under the impression that there was indeed a threat out there, would have acted more covertly in his mission, and may not have attracted Kyle's attention until it was too late!