Check out Amazon's page for the DVD of the Troma film "Combat Shock." Specifically, go down to plot summary. I'll wait.
Apparently, Amazon has a keyword for "Shot Through the Mouth." In case the link doesn't work for you (I had difficulty going directly to the link, and instead had to go through the link on the other page), here are some of the films that match said keyword(s) - Fight Club, Godfather: Part II, Aliens, Full Metal Jacket, Boogie Nights...and that's just their top matches. To be fair, most of the other films on the three pages worth of results are rather, what's the word I'm looking for...shitty. Nevertheless, I support this subgenre.