When I was in the service and in South East Asia, I personally knew an Army SFC up in Hue, who carried a Browning HiPower and a small .32 revolver in an inside the shirt shoulder holster. I kew an infantry Major who carried a Thompson submachine gun that was not army issue. I knew a Navy Lt. who carried a S&W 5" Model 10. Another Navy JG I knew carried a S&W Model 19. I saw him use it to shoot a rabid dog in Hue. Another Navy Lt. carried a Swedish K that he took off of an enlisted man who got it from the body of an adversary killed in a river ambush. (Chicken S**t officer)
On my boat the small arms issue for the crew of fourteen sailors was 2-1911 .45 acp, 2-.308 caliber M-1 Garands, and 2-Thompson SMGs. The rest of the crew manned crew served weapons. We had two twin 20mm mounts. Each mount required a minimum of three sailors to keep it running. I was the port gunner so I was not issued a firearm when the boat got in trouble. I bartered with some indigenous troops for an M-2 carbine. This was my personal firearm. I wasn't supposed to have it. The Craft Master and everyone else aboard knew I had it. When ever we got back to DaNang, I would put it in the Chief's locker so no officers saw it.
I don't believe the .30 carbine round is an effective stopper, but with the M-2 on auto, I could put a whole bunch of holes in you real fast.
I was always told by authorities that personally owned firearms were forbidden in the service. There were a few who disobeyed during trying times for whatever reason.