"OMG!...he's carrying the same gun as me!...how gauche!"
How many times in movies (especially B grade action flicks) do you see an "elite" group of commando types get together for a mission and with say a group of 7 of them there are 5 different weapons and ammo carried? A lot. I know, I know...some of its done to differentiate one character from another into certain cliches...the sniper silent guy....the big burly guy with the machine gun....the shotgun guy...the dude carrying a MP5 on a desert mission because its cool looking and must be a Hollywierd requirement for a gun movie prop... But man, its stupid as hell.
9mm.. 5.56.. 7.62... grenades... 12 gauge... and a guy who likes to throw knives...
Sure the Army has various calibers in its inventory...but its the Army...massive and long supply chains for ammo and repair parts. We are talking 7 guys going in behind enemy lines carrying all the ammo they will supposedly need without resupply. What happens when one guy runs out? So when the cliched Asian guy with a sub machinegun and blades runs out of 9mm..is he supposed to toss in some 7.62 from the tobacco spitting redneck with the M-60? Don't think so. Also, when real soldiers carry different weapons its for a specific purpose, not to look cool.
Lets take a look at this picture of some real life commandos...actually Army "LURPS" (LRRP/Long Range Recon Patrol) from Co D 1-151st Inf in Vietnam. Notice everyone is carrying an issue M-16 firing the same 5.56mm round....smart. Also notice their kit is geared more towards carrying ammo and water than big knives and explosives.
Co D 1-151st LRRPs in Vietnam...thanks for your service guys!
Smarter yet, when these Lurps and Ranger units would sneak 4 to 6 guys behind enemy lines they might even carry a captured AK with them as well. Makes sense too. An M16 and an AK make totally different noises when fired that any soldier on the battlefield can distinguish. So if you are a point man of a 4 man recon team that runs right into the point man of a 100+ strong company of NVA regulars and you fire first the other NVA might be like "whats up? our point man fired at something but I don't hear return fire?" and hesitate long enough for that 4 man group to get out of the immediate area real damn quick!!