Teaching.
Lord_Nikon:
The weapon that was used for BRM instruction when I was in basic training (2002), and the one currently instructed with, is the M-16.
The M-4 is not yet Army-Wide, as such it is up to the individual unit to teach personnel to use what weapons are in the units armsroom.
For example, the combat shotgun is widely used by MP's and Infantry, so they train with it.
The M-9 is also primarily an special use weapon. (MP's, officers, etc.) and as such is not tought during Basic Rifle Marksmanship.
I am not saying that it is a good system, and I wish that all servicemembers had more familiarization with all weapons, but we don't. This is one of the items on my "If I ran the Army.." list.
The fact remains, most US Army personnel will never see official training on the shotgun, pistol, or other special purpose weapons unless their unit has the weapons. Once again I am not speaking for the Marines as I am not familiar with their training standards.
As far as I am aware, as the M-4 makes it's way into the ordnance mainstream, the M-16's will be refurbed and distributed to training units, so it does not appear that there will be much change in the Army marksmanship program.
Another pertinent observation: I saw the combat shotgun used very little in Mosul while I was there, except as a breaching tool. The units that we worked with had breaching team personnel carrying two weapons. The M-4 as a primary, and the breaching shotgun (pistol grip, don't know the model) slung snug and out of the way.
once again this is my personal experience and not necessarily the same throughout the service. It is based on the best information possible from two years of experience and research from the US Army MOS library online.
Edit: After reading the additional posts, just let me re-iterate that, as usual, I am not trying to offend anyone, and apologize if anyone feels that way.
Whether the poster is a Marine or not, NSO's post was very impolite and not in the spirit of civil conversation fostered by this board.
The rest of the info is an attempt to give a little more insight on the current Army Basic Rifle Marksmanship program, and to answer the question asked by Lord_Nikon. I hope it did so and would be happy to answer others as best as possible.