I nominate the British Sten. Parts were manufactured in small shops and garages so there wasn't a factory to bomb. Distributed manufacturing.
One used to be able to purchase parts kits at gun shows forty or fifty years ago. The barrel seemed to be the only difficult part.
Ar15 is like a grown man's erector set. The popularity of them is the near infinite variations that can be made by even a minimally skilled person and one of the widest ranges of compatible "calibers". you can get uppers from 22lr to 50BMG that fit the same lower so you can have the same trigger on multiple calibers, even cross bows. Pretty sure you can also get muzzleloader, shotgun and airgun uppers for it, I just haven't run across those
BP | Bronze Point | IMR | Improved Military Rifle | PTD | Pointed |
BR | Bench Rest | M | Magnum | RN | Round Nose |
BT | Boat Tail | PL | Power-Lokt | SP | Soft Point |
C | Compressed Charge | PR | Primer | SPCL | Soft Point "Core-Lokt" |
HP | Hollow Point | PSPCL | Pointed Soft Point "Core Lokt" | C.O.L. | Cartridge Overall Length |
PSP | Pointed Soft Point | Spz | Spitzer Point | SBT | Spitzer Boat Tail |
LRN | Lead Round Nose | LWC | Lead Wad Cutter | LSWC | Lead Semi Wad Cutter |
GC | Gas Check |