call me crazy but that looks like a thompson with a drum mag, a popular submachine gun used during the prohibition and world war 2 era
Combat tactics were still developing at this point. It's also worthy to mention that supplying an entire army with equipment is much, much more expensive than supplying law enforcement with equipment.
Cops don't generally hang out of their windows with sub-machine guns anymore either. Most street officers and patrolmen are armed with a sidearm and baton. It's why S.W.A.T. stands for Special Weapons and Tactics, because they have weapons and tactical equipment that are suited for special operations, whereas the Military uses weapons that are chosen for combat adaptability in any situation in which recon and combat usually go hand in hand.
The military also has tanks and fighter jets lol