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I have a rewelded receiver with the TNW semi parts installed. The bolt is VERY hard to cock with the TNW firing pin spring and I'm getting short strokes on my recoil cycle. Does anyone know if the original German firing pin spring has enough punch to set off primers on surplus Yugo ammo? Hoping the German spring will free up some energy and still be tough enough to get the job done.

**I meant Romanian 8mm not Yugo ammo.

The TNW-34 f/p spring is very stiff, and causes the internal friction to be very high. If you can hardly cock your bolt by hand, imagine the recoil trying to do it. It cocks against the very-stiff spring on recoil, using up a lot of bolt energy. The gun is hard to cock, and trigger pull is tough. It seems to cause breakage of their fire-control system. The original German spring works well with commercial ammo. The f/a gun drops the f/p when the bolt is closed and stopped. So does the semi. I believe that TNW used this heavy spring to set off hard milsurp primers in the ammo that was common back when it was designed. Their MG34 semi design appears to preclude out-of-battery slamfires. The round is nudged into the chamber by the extractor. In the event of a broken f/p spring or extending f/p, it still "shouldn't" slamfire. Remember, this info doesn't apply to the semi-MG42, in which the combination of soft commercial primer ammo and a stuck protruding f/p could blow open your barrel-door and fill your face with shrapnel. Completely different firing system. Just my observation, I'm not a SOT!...Phil

Good info. Does anyone know if the original firing pin spring will ignite primers on Romanian surplus 8mm?

Most likely it will not fire any surplus ammo due to hard primers. i do have very good success using a orig firing pin spring when using blanks though...get a nice light smooth firing sequence..but thats with blanks only. Doug

Yes, I've also had good luck with blanks using the original firing pin. I've cycled through some primed surplus cases and they fired fine. There doesn't seem to be any reason that the s/a MG34 would have a more difficult task hitting the primer hard enough than the f/a does. The -34 bolt is pretty well "stopped" when it fires in either case, so it's not like the f/p benefits from forward momentum like, say, an MG42. As folks have stated here, the -34 f/p is actually "pushed backward" as it's unlatched in the f/a, contrary the force of momentum impelling it towards the barrel.
Surely the TNW heavy f/p spring insures the ignition of "any" ammo. Just makes the weapon so difficult to enjoy....Phil

I replaced the TNW firing pin spring with the original spring and now the gun runs perfectly just as I hoped. I used 1970's Romanian surplus and didnt get any weak primer strikes. Hope this info helps others.