We pulled three cut receivers out of a pile of lots and found one M2 with sight slots...so I made an M2 ground mounted out of it. The other two were M3 with no provision for sights. Yes, they were intermixed. Also there would be internal differences like the trigger bar and trigger bar stop adjust nut being absent because the M3 used a sideplate trigger... Hadn't thought about this stuff for years.
The M60 is different though, it just rams the cartridge forward and into the chamber. That's why the open link was needed. These extract and run down a feedway in the boltface.
Thanks Jim, I had no idea the rounds were stripped from the rear. Very interesting!