The telescoping recoil spring assembly actually does prevent the bolt from hitting the back of the receiver.
No, it doesnât. Hereâs a slow motion firing footage of the Erma EMP: The Erma EMP at the Range - YouTube. And as you can see, the bolt does hit the receiver at the end of its travel - thereâs a clearly visible moment when the bolt stops and the gun starts moving backwards.
It works with the MP-38, MP-40, and MP-41
Ok i see why it doesnât work, the ERMAs receiver is too short for it to take effect.
Exactly, itâs not the telescoping recoil spring assembly but a mix of receiver length, spring and ammunition power. This design feature is called constant recoil principle, and the guns that show it the best are probably the Ultimax 100 (Origins of Constant Recoil: The Ultimax Mk3 (feat. Mae & a Yeti) - YouTube) and the Knightâs Armament LAMG (Knight's Assault Machine Guns at the Range - YouTube).