I checked the powder charges, they were as expected. The cases in question were all once fired, deprimed, tumbled, lubed, full length resized, trimmed, chamfered and deburred, blown clean of debris with compressed air, primed with a Lee Autoprime, charged with powder that was weighed on an RCBS Rangemaster, charge double checked by weighing case before charge, and again with powder in, bullet seated to 2.800" +/- .005, checked with calipers, and fired in an almost brand new (<300 rounds) Armalite chamber that has never given me any trouble to speak of, besides with the aformentioned low quality Venezuelan surplus ammo. That brass cycled fine after it went through my reloading process.
The necks were not any more or less clean than the factory ammo fired before and after, so I can't really speak to wether obturation is an issue. The only thing that seemed odd to me during loading was that on the initial run of cases loaded with 43.5, 44.0, and 44.5 grains of Varget, there didn't look to be enough room to seat the bullet without compressing the powder. Those rounds were never fired, because the problems occured at 42.0-43.0 gr., and I later pulled those rounds and reduced powder charges to 41.0 and 41.5 grs., which, upon firing, appeared to perform normally.
Based on all of this, the only thing that I can think of is that I got brass from an unusually thick batch.
Now that I have some R-P headstamp in my reloading queue, I may test this theory by loading those with known safe loads of 41.5 grs, and working back up toward 42.0 gr. in .1 grain increments. Sadly, I won't have a chance to put it to the test for awhile, as Federal Grand Jury duty starts next week.
