Weapon, firearm, sidearm, pistol... I tend to switch fairly freely between them. I've found myself using "sidearm" when using it on-duty, and "pistol" when talking about it off-duty. Otherwise I actually just refer to it by make. (I.e. "Let me grab my Sig" or "Yeah, my Glock is in the top drawer, next to the Walther")
I'm in the same boat as (apparently) several other members in that I did my training at Fort Sill, and it was DRILLED into my head that I was using a rifle, or a weapon, never a gun. Guns were for the Artillery and Navy types.