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As per GERMAN MACHINEGUNS by Daniel D. Musgrave on page 326, "Several variations of the 'Ruckstossduse', or 'recoil nozzle,' exist. They differ in the diameter of the hole through which the bullet and the propellant gases pass. The diameter was 11.5mm on the World War II guns; for the same guns this was increased to 14mm after the bolt catch came into use; after the change to 7.62mm NATO ammunition the diameter became 11mm. Later, the recoil booster and the recoil nozzle were made together, I one part."

The smallest Booster-Cup/"Nozzle" that I have that is WWII German "Waffenampted" is 10mm.

Regards, RichardS
US Army, Retired

The heavy bolt requires a different buffer. Without it, the bolt beats the back of the gun to death (stress cracks, separated spot welds for the buffer tabs in the receiver).

The problem is that heavy bolts are available in the US, and the matching heavy buffers are not.