So I have this .223 ammo conundrum, and I'm hoping the reloading collective can lend me some assistance in getting this sorted out.

During the 2012/13 craziness, I bought about 15k rounds of commercially-reloaded .223 in bulk during two of the popular group buys from Precision Cartridge. I eventually received it in good order, and after sharing a bunch of it with friends and others who jumped in with me, I added about 4k rounds of it to my own ammo stash, and forgot about it.

I recently began shooting it during some range days, and noted that about 1 in 15-20 rounds will fail to extract, and will result in a solid in-battery lockup with the fired case stuck firmly in the chamber. This often requires total disassembly or a considerable "mortar" style maneuver to clear them. This has occurred in 4 separate known-good ARs from Daniel Defense, Stag, Palmetto and Sig, with both carbine and mid-length gas, 16", 11.5" and 10.5" barrels. The extracted brass doesn't show any case head separation or other obvious deformities, (although I don't have a fired sample at the moment).

I've narrowed it down to the ammo, so here's my real question: Is there a specific dimension of the cartridge I can check for in this instance that might be contributing to the malfunction? (Shoulder, brass length, diameter, deviation from OAL?) I'd love to just measure and weed out the offenders and pull them so I can re-use the components, but I'd REALLY like to avoid pulling the entire lot if I can, since I'm sure some of them good to go. I measured a sample of 40 rounds out of the lot, and they all varied from 2.219 to 2.232 OAL, with brass at 1.743 to 1.752. Headstamps are a mix of LC, Winchester and PMC.

Additionally, as mentioned, many thousands of rounds from the same lot are out there with some friends, so if I can nail down a solution, it will help many of us.

Any thoughts or wisdom on this? Anything else to look for or consider?