I put military ball through it that cycles my M2 HB with no problem and it still short stroked. I don't believe it is an ammo issue. I was emailed by the manufacturer (I had given up trying to reach them by phone) that it was a bad batch of recoil springs and that a new one would be sent out to me when they got them. Supposedly new ones arrived 3 ago that was my last correspondence with them, I have heard nothing from these people since.
As for the reason behind my rant, I can understand going through something like this if for instance I had purchased a Norinco product, not bad mouthing them just saying for the money you pay you wouldn't be surprised or put out by something like this, but when you dole out large sums of money for an American made product like this Serbu you would expect a little more, either in the way of quality or service. If you bought a brand new Ferrari that always stalled would you whip out your tools and try to figure out what is wrong with it or would you go straight back to the dealership and have it fixed.