For many officers as well as EMs, a pistol is their official weapon and yes, that is what they carry into combat. Non standard personal firearms are not officially sanctioned although in real life, you can carry many different firearms. In Vietnam, for example, I won't say who but I know EMs that would carry sawed off 12 ga. shotguns on patrol. Against the rules, yes (Geneva Conventions?). Effective in jungle terrain, yes.
There is no option in the U.S. Army for anyone to purchase another type of weapon that I know of. Of course if you are a General you can carry whatever you want but then again, I can't recall often seeing officers of that high rank in my normal AO. I think their definition of "in combat" might be different than mine. It was unusual to see anyone higher than company grade officers out in the boonies but I heard tell they were around :lol: