I have no idea what they are- but they look like what they used to call machine guns? Big long black ones with shoulder straps. I commute right past the WTC, where it is common, but other big commuter hubs have them too. We have so many different forces, NY , NJ, and Port Authority, as well as a mix of military these days. I actually think it's the PA that carry the big guns most frequently. I think- but I try not to stare at them!
You mean AR-15 style weapons? They are semiautomatic .223's (5.62mm NATO rounds). Yep, 3 1/1000ths of an inch larger than a .22 you shoot in the Boy Scouts. You appear to, IMO, being mixing up "High Powered" with "Scary Looking".