I've seen the movie "blackhawk down" a couple of weeks ago. There were a variety of small weapons used by our GIs. Some used M-4; some used M16A2 or M-14. What is the regulation to rule the choice of an infantry¡¦s weapon?

Every Army unit has a TO&E, which is Table of Organization and Equipment.  This details how much of what your unit has.  How many people and their ranks and skills.  How many vehicles and types.  How many weapons and types.  And so on.  That's pretty much the law as far as what you're going to get.

You can put in for specific changes to the TOE for mission/enviromental specific things.  If you're mission included something that required a specific piece of equipment, then you would have the TOE modified.  That's referred to as a MTOE, and is what all units actually operate on, because nearly everything is modified for one reason or another.

If it's not on your MTOE, you can't have it.  You can't order it.  You can't support it logistically even if you had it.  That's the way it really is.  

That's pretty much it.  Most line units will have all the same weapon.  Some units get more latitude, and some have different types because of different missions.  Special Ops units get a wider variety and choice of what they have available because of mission requirements.  Still, even then there are budgets and they don't just get whatever they want.  They just get to choose from what's available (which is a good selection to choose from).

So most of the Army rifle companies you'll see will have M4s OR A2s.  Usually not both.  Special ops will mix and match more.

The other services have different ways of doing things, but BS is pretty much the same in the DOD.  They may be called something different, but I'd venture to guess the USMC has a similar way of doing things.

Ross

Just to add to what Ross said, Deltas have a wider latitude on what weapons they use, and generally they can add aftermarket accessories like triggers and optics that aren't available to regular troops. ICBW, but I understood that Deltas can go outside their MTOE to get stuff they want. Delta Randy Shugart was the only one in the movie (and the book) to use a M14.