"By the end of the exercise, we'd broken out the old M38s out of their lockers. Damn good gun the '38. You could take a dump in the mechanism, bury it in sand, dig it out three months on, tap out the crud and it would still fire first time!"
The M38 is known for being extremely reliable (far more so than early M56 Smartguns). Supposedly the M38's mechanism could be filled with crud, buried in sand for three months and after tapping the crud out of the mechanism, still fire the first time.
During the early days of the M56 Smartgun, the M56 was extremely unreliable and jammed commonly. This made Recruit Weapons Instructor Gunnery Sergeant Ron Huffey use M38s for exercises instead of the M56.
In Aliens: Colonial Marines, a gun known as the M39 Submachine Gun appears. Wether or not M39 is supposed to be a successor to the M38 is unknown. However, if the M39 was intended to be a successor to the M38, it could be a reason why the "M3A" designation seen in M39 Submachine Gun's concept art was changed to "M39".