QPQ is the technical term for melonite, Tennifer or Tufftride. Its basically Nitro carburizing case hardening. Not all QpQ is the same though so be careful if the polishing is not done well most of the advantages in QpQ will not be there. Also QpQ should not replace a real heat treatment process and I fear some manufactures are using QpQ as a heat treatment process alone. Also QpQ on a bolt head could cause enbrittlement issues and the only way around this is to lower the temperature used in the process which makes the process less effective.
However our main issue with using it on a bolt or carrier is that the blackening wears very fast in an anodized alum upper and while the nitro carburizing case harden effect is still in the metal even after the outside black finish is gone we feel people may not like the fact that the black finish color wears off so quickly and because the black finish is so smooth the finish wear is very noticeable. While the nitro carburizing case hardening is still in place and this does provide great corrosion resistance we believe customers may not be happy about the outside black wearing away which like I said happens fast when rubbing in a 60-70rc anodized alum upper.
So therefore we believe QpQ/Nitride is not a good finish for a bolt, gas key and bolt carrier, though we believe it is one of the best processes for barrels.