I am however, training to use state of the art equipment. Improved since the days of all these grizzled Nam vets. They have been made to be more weather resistant.
Assuming you're referring to the M16...I don't know who told you the M16 (A1 or A2) is "state of the art". An M16A2 made by FN yesterday isn't any more modern than an M16A2 made by FN 10 years ago. The local ROTC unit here doesn't even have A2s, they have A1s.
I've carried an M16A1 that was so old the receiver was marked "Colt AR-15", putting the manufacture date sometime in the 60's. I think that was probably the only original part left on the rifle, but it's hardly a "new" weapon by any stretch of the imagination.
The worst environment for use of a weapon would probably be desert. Fine, talcum-powder like dust can get into every pore, every nook and cranny, and does so everytime you kick some up (anyone who's ever ridden in the back of a track in a dusty environment can attest to this). Fine grit can be VERY hard on a weapon; it can get in between the moving parts and gum things up pretty bad.
Cleaning can get the grit out, of course. But when you're riding in your track on the way to your engagement area, getting covered in dust from the trip, you're really not in any position to tear your weapon apart and clean it (not to mention dust sticks to wet lubricants like crazy).
The M16, in my experience, does okay in dusty environments, but it's important to keep the dust cover closed, the barrel capped and your magazines covered. The SAW works a little better being a somewhat mechanically simpler design (with a better gas system).
Cold environments (VERY cold artic type environments) present problems all their own. For one, from the human standpoint, operating many weapons with thick gloves on can be difficult. If you don't believe me, buy some thick winter gloves (you folks that live down south, and consider forty degrees and rainy to be winter might have trouble finding them) and see if you can even get your finger in the trigger loop of a typical pistol.
That notwithstanding, very cold environments also require special lubricants that won't freeze, and you have to be constantly trying to keep your firearm dry, as any moisture can freeze.
I would have to say that outer space is the worst environment. The extreme temperatures can't be good for weapons (try from near absolute zero to several hundred degrees), and even a grazing hit anywhere on your body can be big trouble if you're unable to seal the rupture in your space suit...