I own a TNW semi-auto MG34. Having shot that gun I was giving it a good cleaning. After taking off the flash hider I separated the "booster cone" and gave it a good cleaning. Reflecting on the gas pressures and heat that are acting on this part I was amazed that it cleaned up so well (still nice and shiny and smooth). I was curious to know more about the "booster Cone" and so I grabbed my copy of the Folke Myrvang book on German machine guns. He described that the "booster cone" should have an 11MM hole in it, but rarely you may encounter one with a 9.5MM hole. The 9.5MM version is only to be used with an aluminum cored 8X57MM training round. The smaller hole makes the MG34 action too "violent" in operation when used with "service" 8X57MM ammo. This causes undue stress on the moving parts.
Guess what size hole is in my "booster cone"? Now I know why this gun really throws empty brass out so hard!
Anyone with a TNW semi MG34, or has a live MG34, or has built a semi-MG34, or has a parts set thinking of building one, I STRONGLY suggest that you check the hole in the "booster cone" with a micrometer. I will be replacing mine before I shoot it again!
"Ake Ake Kia Kaha"