In the military, the pistol is a sidearm and not the primary weapon which is normally a long arm of some type/configuration, i.e. M4, G36, SAW, etc.
In law enforcement, the opposite is true, with exception of the extremely small percentage of time that officers spend in SWAT related duties where they typically have a long-arm as their primary.
That being said, if I'm carrying a shoulder fired weapon; it's not gonna have a pistol cartridge (either .45ACP or 9mm), and will be a .223 or 6.8 or .308.
Shoulder fired pistols (that what I call submachine guns) have lost a tremendous degree of popularity in law enforcement of late as most departments have figured out that despite it's "tacticool" and sexiness of the MP-5 for example, Bruce Willis' weapon of choice in all those cool Die Hard movies, the terminal ballistics of a pistol cartridge absolutely pale in comparison to rifle cartridges. Carbines have equal or better specifications than SMGs when it comes to overall length, weight, etc. and the over-penetration concerns of rifle caliber weapons in an urban environment have long been dispelled as false (ammo type dependent; ball is bad in an urban environment, jacketed soft point is good)...
So when real, no shit operators, both in the military and civilian law enforcement do choose a caliber for their sidearm, what is it normally? .45ACP, period. LA SWAT, CAG, FBI HRT, etc., etc., etc., all rely on the venerable .45ACP in a 1911 configuration for a reason.
