I reworked the bolt by welding a plate on the bottom towards the front of the bolt carrier and filing down the bump at the rear. The result is that the hammer, when held by the disconnector, only touches the bolt where this plate is, and has a lot more time to float and settle on the underside of the bolt before this raised surface has moved by. See pic.
I went to the range yesterday morning and began loading single rounds into the belt. Fire, hold trigger, release trigger, listen for reset. I ran through a dozen or so rounds this way and not a single occurrence of hammer follow!
Load 4 rounds into belt. First squeeze...bang. Second squeeze...bang, bang.
Open top cover, eject forth round out of chamber. Notice a fairly generous indent in the primer of the ejected round. I look things over a bit and fire a couple more singles...no hammer follow. Load two rounds into belt. Squeeze trigger...bang, bang.
Recover the two casings and one had a noticeably smaller primer indent.
So is this a firing pin bind condition or a firing pin inertia condition? Or just soft primers? I've been shooting PPU 8mm Mauser ammo (Serbian made by Prvi).
At one point I had added washers to increase the firing pin spring preload in steps until I didn't get ignition and I've been running with one less washer ever since so I woudn't think it's FP inertia.
I did notice some scuffing on the FP itself in the area of the wedge. Looking at the wedge, it looks like the hole was drilled out and there is a little mismatch in the hole like it was drilled a little off.
I also noticed that when I take just the bolt head and the FP, there seems to be a fair amount of FP wiggle up until the last ~.03" of fully seated which then it becomes pretty solid and doesn't take a lot of side load to bind. Should the FP be polished down a little on the first two diameters to increase clearance to the bolt?
How about the two piece firing pin? Is that something that can be retrofitted in the my existing single pin parts?
Baby steps. This rifle is awesome and has been a lot of fun to figure out!