The QPQ bcg craze hit soon after the NiB BCG craze. It has a lot going for it, and a very long history of being used to treat metals for various hard-use applications.
Since it came out in vogue for bolts in BCG’s (JP used it for a long time in carriers, but I believe RCA was the first to treat bolts), I have also noticed that several who used to use C158 for bolts, now use 9310. Is this because their suppliers changed, or did they note that the 9310 performs better when QPQ processed? I do not know. That is beyond me.
However, I have yet to see either in person, or on the internet, a broken 9310 QPQ’ed bolt. I have seen some that have gone well past 25K rounds, with some reported at 50K+, but I have yet to see a picture or hear a take of one that broke.
The only broken bolts of this type that I remember reading about were a bad batch from ARPerformance ones that Harrison quickly either replaced or pulled from circulation awhile ago. If memory serves, it was a bad heat treat that was the culprit. Apologies as I do not personally have any experience with them though.
I do have a 9310 phosphate bolt in my frankenbeater with around 2k rounds on it though that so far has been performing just fine if that helps at all.
I don’t think the ARP bolts are QPQ nitride treated - I think they are just phosphate coated. I could be wrong, but I have two and they don’t look like QPQ.
I recently picked up an AIM 9310 QPQ bolt out of curiosity. It looks pretty. Haven’t used it yet. I am wondering if a nitride treated bolt is likely to wear out the lug area of the barrel extension faster than a phosphate coated bolt would.
From what I understand is that 9310 takes the alternate treatments and coatings better than C158. Ive put 25k rounds through two barrels this year on one CVD 9310 bolt so I’ll take a closer look at the barrel extension.
An AR47 I’m currently building has QPQ barrel, barrel extension, and bolt. BCG is NiB. I hope it lasts well.
I agree that an extra hard bolt on a normal barrel extension might cause some problems, but don’t have enough experience to say either way. What’s the typical finish on barrel extensions?
I’m not an engineer, so I am muddling through this. The barrel extension is plain steel (often stainless?) with a hardness of maybe 20-25 Rockwell C? A typical bolt would be hardened steel around what, maybe 40-45 C? And the nitride treated bolt will have a surface hardness in the 60’s. The nitride treatment makes it harder and slicker. I imagine it also means that any grit between the two will be wearing primarily on the extension lugs since the bolt is much harder. This is beyond my understanding so I am really interested if any of the engineers present can comment on how this works.
But is that yield strength loss offset by the tougher surface? Most cracks begin in the surface of the rear of the lugs/fillets, or the cam-pin hole, and progress. Does the QPQ treatment help with this? Is the lower CoF of it in the barrel extension (real or theoretical?) help with the axial loading of the bolt lugs?
There is a lot more to consider aside ultimate yield strength.
I am not vouching for the above. I am simply posing the question(s).
I agree. It would be nice (and cheeper) to be able to pit bolts of the various compositions and processes against each other and HPT them until failure and declare the last one standing the winner, but that won’t really give you the real idea about which will last in a carbine. The variables of heat, fatigue life, friction and gauling, etc., are not accounted for.
Yeah, fatigue life being there most complicated and interesting aspect to me.
Also, I wonder how cryogenic stress relieving/treatment, like they do for many automotive and commercial machine parts, would play into this bolt life equation as well. V7 Weapons offers a cryogenic treated C158 bolt in phosphate. Hmmmm.
I tested a NIB bolt and the base metal on the lug was in the 50s Rc. I don’t know if it was carpenter or 9310.
20s sounds way too low for an extension, but I’m a ChemE in the steel industry, not a metallurgist, so I could be wrong. We don’t deal with ss much at all.
