Imminent case head separation?

[Yaremkiv]

Hello reloaders of Rokslide. I'm developing a load for my 6 ARC Ar15 and I ran into a problem. The 1st shot was fine. The 2nd shot left a ring at the web of the case. I cut it open and it looks like the wall might be thinner in that area but the BTSD measurement only increased by .005". The brass is on its 2nd firing. It's first firing was as a factory round from Hornady. The charge was only 25.8 gr. of Varget shooting a 103gr. ELD-X. The unsuported portion of the case, just above the extractor groove didnt grow by even .001" after being fired. The case extracted and ejected without a problem. Primer pocket didnt loosen up. No ejector marks on the brass either. I'm relatively new to loading for gas guns. What am I missing? I only bumped the shoulders back .002" after its initial firing. Could my Ballistic Advantage barrels chamber be cut too long?

 

[RockAndSage]

I don't have much to offer on the reloading side of things, but on the AR side of things, the gas system is something to consider carefully.

Short version: the higher the gas pressure at the gas port, the more you get things like case-head separation, broken extractors, broken bolt lugs, and a number of other issues. Essentially, because the BCG is being forced back before the bore has depressurized enough for the brass casings to pull back away from the chamber walls.

In Vietnam, there was an early period of common fielding of the M16, where they were using surplus 30-06 powder, IIRC, and it caused exactly these issues - because the pressure curve of the burning powder was spiking too close to the port. Slower burning powders peak in their pressure curve further down the bore. Lots of broken bolts, extractors, and separated case-heads.

You get similar things in improperly built SBRs, especially ones below 11" with short gas-system lengths. The longer the gas system, the less of an issue this is, as a general rule. You can also tune gas pressure/volume through things like adjustable gas blocks, or gas tubes with smaller ports from Black River Tactical. This is especially helpful if you're also running suppressed, as cans slow down the depressurization of the bore as well.

To be clear, I don't know if this is what's happening to your AR load. But, because this is a thing, as you're going though your diagnostics, be sure to ensure your powder is appropriate for an AR or your gas-system length, and not just bolt guns.