As the other post to this thread suggested, there are several other things (challanges) that may be in play here.
Some of these are associated with any of the moving-parts slightly "dragging" to any amount during operation......some of these are:
1: Barrel movement in barrel-jacket.
2: Recoperator-rod/spring inside lower-part of receiver not allowing full barrel movement to the rear during operation.
3: Bolt-lock (for lack of correct nomenclature) possibly not fully seated or dragging on bolt-roller as barrel-assembly with locked bolt move to the rear.
4: Ejector-rod dragging on the inside of receiver......or bolt-ejector-rod slightly bent.
5: Other parts of the semi-only firinmg-mechanism dragging on bolt as it moves to the rear.
6: Buffer needing adjustment to a slightly shorter length.
7: Inconsistant ammo loading.....try known good ammo that is not more than a few years old to weed out ammo issues......but remember to keep the ammo at/near the same loading for WWII German ammo.
8: Bent barrel-jacket.
9: bent and/or dragging feed-parts inside the top-cover or a top-cover that is slightly bent downward.
10: Worn barrel-bushing and/or forward area of barrel that has the outside grooves worn......where you lose part of the rearward gas pressure during firing.
These are just a few of the items to check.....there are others and I am sure that several Board-Members will add more.
Remember that anything that feels like it is "dragging" will use up part of the recoil "pulse"......and these need to be corrected.
Anyplace that has worn parts at the muzzle that are acted upon by gas=-pressure need to be checked.....replace worn parts.
Regards, RichardS in Bay City, MI
US Army, RetiredBlanksguy2001@chartermi.net