I had two tours in Vietnam as an USAF pilot, flying the Bird Dog, O-1E (L-19 in Army parlance.) The first tour was as a Forward Air Controller with 5th Special Forces out of their B-camp at An Loc, lll Corps. My issue gun from the Air Force was a S&W M-15, Combat Masterpiece, and the issued rounds were 130 gr FMJ, .38 Specials. Soon after arriving at An Loc, I locked it up in the Team vault, and bummed a 1911A1 .45 ACP from the Berets. For the bulk of my time there, that's the gun that I carried, slept with, and it was only off my person when I was in a shower point. For a brief period, I carried a Browning Hi-Power in 9mm, just for its magazine capacity...
Personal firearms were officially discouraged by the Air Force, and getting one back home after your tour was over, was problematic...I left mine at home and carried issued arms while I was there.
I saw several .357 Magnums while there, two S&W's and a Ruger Blackhawk of all things. They were carried by senior Army officers, L/C and above, and BTW, the guy with the Ruger was killed in action during my tour, though I don't think another gun with a more modern action would have saved him...they were overrun in a night attack.
Best Regards, Rod