The Soviet Hind D attack/transport helicopter have a canvas covering for its tail; it was light, cheap and lets cannon rounds pass through without detonating.
Back to sheet metal works, there were a few "tube" guns that evolved into sheet metal;
The British Sten MII was further simplified from Tube to wrapped sheet steel, the MkIII was wrapped around a mandrel and welded along the top.
The German MP-40 , from the milled tube MP-38, was sheet wrapped around a mandrel and welded in the shortest places, along the seam of the cocking handle slot.
The Soviet PPSH-41 was sheet from the start, but the design inspiration was a simplified milled Degtarov. The PPS-43 was sheet from the start
The US M-3 "Grease gun" was stamped in 2 halves and welded.
These were all low pressure guns, of simple, unlocked design.
The real trick was mating a sheet metal "body" with a milled (strong) Trunnion to make a receiver that could take large amounts of barrel and lock pressure, heavy vibrations and yet be light and strong.
Shorter ,less powered cartridges at that time were not new, they were just not what the military expected to be usefull at long ranges.
The German military's tactics were developed and the weapons caught up, not the other way around. It wasnt untill mid WWII that shorter ranged cartridges that out performed pistol sized cartridges but didnt need full power were accepted.
If they had the AK in 5.56 in '33, they would have definitely loved the gun, but , like the US Military, it would have taken alot of "proving" to get the cartridge to even have a second glance given to it as a military round.