I had a partial case head separation using a 1.8oz buffer

[LRRPF52]

Playing devils advocate here... if it were an in battery blowout, would the resulting minimal damage be attributed to the short barrel length providing a quicker drop in pressure as bullet exits muzzle in a relatively quicker time?

Conversely, I'd we extrapolate this failure to a 20 inch barrel, would we expect to see more damage due to slower pressure drop off from the barrel length?

No. Chamber pressure doesn’t know whether it’s hitting peak pressure in a 10.5” or 28” barrel.

Curve ball: Some will point to gas port location, which is one factor I’m pointing to in this failure, but my perspective is that gas port location led to early extraction while the case was obturated/trying to stay sealed to the chamber walls. Gas port isn’t enough to lower pressures significantly though, as plug dwell time is still present.

Muzzle velocity differences between gas shut-off and gas open are minuscule, sometimes in single digits. The near-miraculous energy imparted on the projectile takes place very quickly due to the efficient converting of the propellant mass inside the chamber. The resultant expanding gas is what propels the projectile down the barrel. Slower-burning ball powders have been shown to have significantly higher port pressure though, as well as higher muzzle velocity. This is one of the main early lessons that the AR-15 experienced with the change from stick to ball powder in the early batches of M193 from 1959-1965.

What was one of the main problems encountered? Excess cyclic rate, partial and total case head separations, frequent malfunctions in rifles with tight and corroded chambers, using the lightweight Edgewater spring guide for the buffer. Adding the heavy rifle buffer and chroming the chambers were 2 of the main things that fixed the problem, along with maintaining better consistency on reamers. The 1963 production Colt 602s were made with reamers that were allowed to run ragged, under-spec, so chambers got too small.

Brass trying to adhere to the chamber while the extractor is trying to rip it out at excess carrier velocity....torn rims, partial and full case head separations.

This is why I do what I do with 10.5”, 11.5”, 12”, etc. CLGS AR-15s. I use proven methods and components to slow down the cyclic rate, number one being use barrels with gas ports cut within spec (smaller).

Next is to use adjustable gas carriers that choke the gas before it can flow unrestrained into the expansion chamber.

If I didn’t do these things, I would expect to see brass being stretched and case head separations from too fast carrier velocity.

I also remove the O-rings from any bolts that come my way.

Those were a band-aid from Crane to fix excess carrier velocity problems with 14.5” suppressed M4A1s and suppressed 10.5” CQBRs, especially the initial batch of SOCOM barrel profile M4A1 “upgrades” that had tight chambers out-of-spec.