I was a tank commander on a M48A3 tank with the 77th Armor in Vietnam on the DMZ. OEM equipment was 2 M3A1 SMG’s per tank. The M3 was one of the finest and most reliable weapons I ever used. I dropped it in mud with the ejection port cover open, to see if it would still work. I scooped the mud out of the chamber with my little finger so that rounds could enter it. I did nothing else. The thing went thru two magazines of continuous 30 round bursts, ejecting mud along with cartridge cases. It was practically self cleaning!!
The bolt ran along two guide rods that were located in holes at the back of the stamped and welded receiver amd by a clipped on trunnion plate in the front. That and the very low cyclic rate of fire (450-500 rpm) contributed to it’s smoothness and controllability. It was ridiculously easy to maintain and repair. I think that penny for penny, it was one of the best SMG’s ever designed. I could easily keep every round of a 30 round burst on a man sized target at 50 yards.
The only weakness was the mag, being a double row single feed position type. I cured that by pressing two mag springs together and stuffing them into the magazine tube. It NEVER failed to run perfectly with that setup. Zero stoppages.
My father told me the M-1 Carbine was a beautiful piece of engineering and craftsmanship but in reality it was a pain in the ass to maintain under the conditions he found himself.