I've got in my computer a number of files that were used by the UFO2000 project (a multiplayer version of UFO) which include the sprites and inventory weapons on .png format for the following weapons (includes ammo clips):
*BA50 rifle
*M-16 assault rifle
*AK-47 assault rifle
*M-4 carbine
*FAMAS assault rifle
*G-3 assault rifle
*MP5 SMG
*SA80 assault rifle
*SAW LMG
Plus, there's also graphics for bazooka, M16 + M203 Grenade Launcher, Claymore mines, Glock/P227 pistols and a couple more.
I wasn't the designer for these images and they were only authorized for use in UFO2000 and I don't have time to implement them but if someone is interested I can give the email of the guy who originally designed them for you to contact, and if he authorizes, I'll share the files.
Any takers?
EDIT: meanwhile, this has been posted elsewhere and it seems you don't need to ask permission as long as you credit the images.