Re: temperature-- the bolt lug difference between piston and Di is about 40*f. The bolt body, around 100*f. Now, this temperature cycle fatiguing the bolt, would it not be much worse if another 50*f were stacked in? Would it then not be logical that an m4 shot in the winter would break the bolt at a different rate by a very noticeable amount of rounds than one shot in the heat of the summer? I just can't believe such a differsnce is measurable.
Re: qpq-- I had the slide on my stainless Sig qpq treated. I also have a stainless Sig that was not. The one that does not, shows self limited peening where the barrel hood impacts the locking interface of the slide, fore of the chamber. The qpq weapon was fired a few hundred times, then sent to be treated. Upon return, and firing it, I note fine chips instead of peebing of the slide. The barrel lug on the other hand, I have to inspect again, but it seems much more suitable. I would not qpq things blindly and expect positive returns in all cases.


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They both work well for the vast majority of users.


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