Originally Posted By: alfReading between the lines, the barrels are from different manufacturers, so that could possibly be the difference. Some barrels will/can allow you to run more powder. Case in point, one of my 22-204's is a 5R, and I can run a half grain more powder in that barrel than my other 4 grooves.
Another thing I've seen is that custom actions may/can allow you run more pressure than a run of the mill factory gun. An AR doesn't have the tight lockup of a bolt gun with a relatively massive 2/3 lug design, which I believe is part of why the pressures are limited to the 52K range versus another 10K+ in bolt guns.
There's such a thing as the barrel not being indexed properly prior to chambering, and the runout at the base wallows out a fatter web dimension, letting the pockets open up. Measure fire brass at the base with a good set of micrometers to compare.
What ever the reason, the web is expanding more than your bolt gun is, and is expanding the primer pockets, which normally is a pressure problem.
Nothing special on the rifling...the bolt gun is a Muller barrel and the AR is an Xcaliber...neither are 5R or Polygonal rifled. From what I am getting on headspace measurement wise between the 2 rifles is that the AR has about .002 more headspace than the bolt gun which isn't really bad when the rifles were done by 2 different gunsmiths with the same reamer. The bolt gun is a Rem 700 action, so not custom.
I did re-adjust the gas block today doing some experimenting and I think the AR might have been over gassed. I took 3 pieces of new brass and started out .5gr below my bolt gun load. Closed the gas off and fired the 3 pieces of brass about 8 times till the bolt would cycle properly and lock open on last shot. Primer pockets remained as new on 2 of the 3 pieces and the 1 loosened slightly but was still good. This gas block is designed to be adjustable with a throw lever for running suppressed and non-suppressed, so I then loaded the same 3 pieces of brass 3 more times getting the gas block set for suppressed...all 3 primer pockets stayed the same...then went to firing suppressed and non suppressed another 4 loadings on the same 3 pieces of brass and they are still going strong. Everything on this build was new and I think the initial settings on the gas block needed adjusted some more after the rifle got wore in some...and I may not have had it adjusted very close either. For now it seems to be doing fine with .5gr less than the bolt rifle...I can live with this...especially with the brass holding up to 15 reloadings and still staying good on primer pockets. I will experiment with bumping the load up a little closer to the bolt rifle load and seeing what it does, but for now it seems to be OK where it is. I really think I had to open up the gas more when everything was new and as it wore in it was getting too much gas. I now go only 3 clicks on the gas lever from suppressed to non-suppressed and it originally took 5 clicks to fully function both ways.
Thanks to all for the insight...appreciate it very much.
Gene