Reduced diameter booster cone
Useful part for a weak-cycling 8mm gun or 308 conversion.
The MG34 is a Short-Recoil operated gun. When the bullet reaches the muzzle, the recoil and excess propellant gases enter the booster area and drive the barrel rearward against the mainspring and barrel return piston (recuperator). This movement forces the roller lugs on the head of the bolt against the cams to unlock the bolt and allow it to reciprocate in the body of the receiver. The 308 cartridge is slightly weaker & lighter than the 8mm cartridge and more cycling / booster pressure may be necessary to properly cycle the gun. The 308 barrels are slightly lighter than the 8mm barrel, so less recoil + booster force is necessary to cycle the action.
The 308 booster cone has less internal volume and smaller diameter exit hole for these gasses to expand. These shapes restrict more pressure in the booster assembly compared to the 8mm booster thereby driving the barrel rearward with more force. Typically you don’t need to use our reduced boosters unless you are shooting some weak 308 ammo. So use this part only if necessary to increase cycle pressure.
To determine the amount of booster pressure you’ll need to properly cycle your gun, you should fire some single shots with your standard 8mm booster cone to see if your gun cycles properly in 308 with your 8mm booster cone. If the gun fails to eject the spent case, more pressure is needed to cycle the action and the 308 booster cone should be used. Once installed, retest. If the system is now cycling too hard as can be felt by the shooter and seen in majorly deformed spent cases, the overall pressure in the system you can be reduced by loosening the flash hider 3-5 clicks by lifting the spring loaded lever.
A Note on Extractors:
If the gun functions fine in 8mm but fails to extract and eject in 308, the problem is likely the extractor. The 308 case has a slightly different shape than the 8mm case. Strong extractor springs are necessary to properly hold the 308 case firmly in the bolt face until ejector contact is made to throw the spent case through the ejection port. In some cases a step needs to be machined on the extractor to better ‘grab’ the rear of the 308 case.