Way Of The Gun
Can't believe nobody mentioned "Way of the Gun", one of the best gun-handling movies ever made, in my opinion. The director, Christopher McQuarrie, had a former SEAL (his brother, IIRC) train the actors, and choreograph the gunplay. There are several scenes where characters use military-style small unit tactics that are pretty well done.
Don't know if it could be called definitive for a particular gun, as there are a great many and varied firearms, but the grand finale firefight involves two guys with 1911s and a whole lotta mags, duking it out with a dozen or so guys with shotguns and snubby revolvers. It's not exactly definitive, but it's certainly symbolic.
Weapons involved in the movie include (but are not limited to):
H&K USP
1911
S&W J-Frame (or earlier models)
Remington 870
Galil .308
I love this movie.