Looking at my bolt and stock, the material could have been cast thicker via projected forward or the inside sharp corner cast w/a 45 degree angle or at least a inside curve. Cracks develop easily at inside square corners. Add that the recoil spring pressure is not even inline w/t cross section, but offset which adds leverage pressure at the bottom of the inside corner!
One good thing, assuming the bolt & frame are the same material, is that the material seems to have a little elasticity. I've cut on my frame quite a bit via a bandsaw and when I got to the last of the cut I stopped to break the piece off and it takes quite a bit of bending before it snaps off. Cuts rather easy too vs a hard brittle casting.
Most likely the break is just a fluke. If there were hundreds of broken bolts, HP would probably want to make a new casting design vs a rare replacement cost.
And this being a 9mm, the real test is when the 10mm's get shot a lot.
No, I'm not done!!
Better yet, if you're going to have a full stocked carbine, why not just build a captive recoil spring system and buffer right into the stock? You could have 1 or 2 rat tails like a FAL carrier that push against recoil springs.
Although I personally chose to work w/t HP carbine as I had the option for a folding stock or Bullpup conversion.
