Well, it's my opinion that with the myriad weapons already available, there wouldn't be much point to have a pile of new ones that are the same as old ones with different graphics. Unless you were going for a theme. So a few with a bit more edge would be warranted.
Not sure exactly how to make it happen, but some upgradeable weapons would be cool. Perhaps at one point you find an optical scope that someone could "attach" to a sniper rifle to give it night sight and/or a small accuracy bonus. A harness could be added to a big, heavy gun to lower the ST requirement.
A ghillie suit, with low protection but good camouflage.
Grenades. A stick grenade could be thrown farther than a regular grenade, although it would weigh more due to the handle. A phosphorus grenade could have a critical hit bonus, since inhaling the chemicals is lethal. Right now, playing with anything besides guns isn't very effective, so other weapons with more potency could broaden the player's options a bit.
Bear in mind the closest I've ever been to military weapons is in Fallout. That said, how about some weapons with an extra mode of attack? During (I think) Vietnam there was a variation on the M2 built that had a single-shot mode to be used for sniping. The pulse rifles in Aliens had three modes; single, four-round burst, and fully automatic.
More ammo can be added. According to Wikipedia, 7.62 bullets have both AP and FMJ varieties.
Another thing that could be messed with a bit: In the wasteland, food is a pretty precious commodity, so certain types of food would have a significantly higher trade value depending on the shelf life. Packaged military rations that won't go bad would be quite expensive.
Likewise the non-radiating/non-rotgut alcohols (a bottle of genuine Jack Daniel's - wow!) and if you really wanted to give the player a treasure, put in some water. Heavy as hell but worth some serious bucks.
I've often wondered why Fallout is awash with booze and drugs, but no smokes. I'd personally think cigarettes would be used as money, and some people would kill their mothers for a half-decent cigar.
Granted, money is kind of a moot point, so trade goods like this are mainly for atmosphere. But atmosphere sounds like what you're aiming for. Anyway, just a couple ideas off the top of my head.