A phased plasma rifle in the 40 watt range would actually exist in this setting, and "only what you see pal" would actually be a serious response.

(Also, it you're not smart enough to not leave live ammo lying on the counter, at least be smart enough to hire a T-Doll to be a clerk.)

http://www.imfdb.org/images/b/b1/TerminatorModel15.jpg

(foreground S&W is replaced by the movie's AMT Hardballer)