As a Navy FMF Corpsman, I carried a Colt .45 on both tours. Also had a .38 stub nose as a backup (passed on from another Corpsman when he rotated home) but never used it. Had to get ammo from the civilians (RMK/BRJ Construction guys). I too passed it on when I left from my first tour.
Bought my first .45 Colt for $45 USD in Viet Nam on the black market. Was on Swift Boats and they did not issue side arms except for a .38 revolver the boat officer carried.
I was on the boarding team and had to jump from our boat to some rickety, slimy, filthy wooden boat often in heavy seas. Finally got tired of trying to do this with an AR15 so bought my own .45. Shot the heck out of it and sold it to another Swifty when I left...for $45 bucks.