It was on a ship, 5" naval gun. We had a platoon of Marines onboard and they were given a 5" mount to keep them occupied. I didn't see it happen, as I wasn't in the mount with them, but it was the talk of the ship for a few days. Somehow they got the projectile in backwards, placed the powder casing behind it and rammed it home. I said earlier they closed the breechblock, but after thinking back 45 years I remember they rammed it home, may not have closed the breech, may not have been able to. They were in the gun mount, I was in the gun director above them, so like I said, I didn't see it.
It wasn't a Marine thing, LOL, could have happened in any of the six 5" mounts, just happened in theirs. We got in quite a few shootouts off the coast of N. Viet Nam from having to go in really close to reach targets well inland. We'd get up in a bay or river mouth and they'd open up with shore batteries from all directions, then we'd have to shoot our way out, often firing port and starboard. It was a real adrenalin rush and a good situation to fumble occasionally.
As to what unit, it was the U.S.S. Newport News CA-148. A heavy cruiser with nine 8" guns in 3 turrets and twelve 5" guns in 6 twin mounts. We could reach 22 miles inland with the 8", and like I said often had to get in real close to reach bridges, trains, convoys etc that were well inland.
Any more questions?
