Case Bulge

[LR1955]

Happens with plenty of other cartridges and actions:

LR-52:

The top pictures look like someone used a .308 sizing die on a 30-06 and bulged out the base. The bottom ones look a lot like a sizing die not sizing down the full length of the brass.

I have never bulged out the base of a cartridge of any caliber so the base looks like a tire. Sure it wasn't a sizing issue?

LR-55

[LRRPF52]

The top one from was from some type of 9mm AR-15 after firing.

Not sure what the bottom one was from.

[oyster]

The new 6.5 Grendel Belted Magnum?????

[Former Cav]

I always thought that ring around the bottom was from incorrect headspace. Am I wrong or is that also another cause. I had an FN/FAL kit gun do that until I bought a new barrel and had it headspaced by a PROFESSIONAL with a track record.
BTW....I've had S&B Brass in my 45 ACP SPLIT all the way down the side....FACTORY LOADS. I sent them photos, they sent me a free box of 50. The only brand I've had to that and I've had this 1911 for over 25 years or so. Never a problem with other ammo or my reloads.

[Klem]

Cav,

Yes, there are other reasons for case bulging, as seen in the photos. Headspace can be fine however the case gets dragged out before the pressure has dropped, like in 52's top photo. In that it was direct blowback (no gas system) using handgun rounds. The case is not locked in place during firing but held by a big chunky bolt and spring, like an Uzi or Sterling.. The bolt and spring holding it in battery was not strong enough and they pushed back too early, or the bolt bounces in full-auto (timing).

You are thinking of case separation caused by excessive headspace. This can occur the first time you fire it but usually it is 'incipient'. So it occurs slowly over multiple reloads. Stretch mark signs of incipient case separation are usually further up the case where the brass is thinner. I check for this using a stretched ou paper clip with a hook bend on the end. Scrape it on the inside and if you can feel a trench discard the case and check your headspace.

When sizing cases with tight dies like small-base they can look a bit like 52's lower photo. The case gets squeezed but the solid web is less affected. The Lee brand in Grendel is also a pretty severe squeeze.

Hot loads will also cause the solid brass down the web to extrude and fill the space in the bolt face.

[LRRPF52]

A theory I have is that you can experience this with a combination of early unlocking and undersized brass, like almost all factory ammo is.

Residual back pressure from the bore continues to exert force on the hot brass that just had 47,000-62,000psi (depending on the cartridge) lit off inside of it, so it naturally expands to the shape of the chamber with the case head being forced against the bolt face (bolt thrust).

If the cyclic rate is too fast for the action, the bolt carrier will be traveling too early to the rear and unlock the bolt earlier than ideal, still with enough pressure from the plug dwell to yank the spent case out too early. The brass around the case web then expands into the void a little as it is extracted from the chamber too early.

Yes, I know the projectile has left the barrel before carrier momentum is initiated, but carrier momentum seems to be initiated early enough to still allow the brass to expand outside of the chamber with early extraction.

We know the chamber is not containing the area around the web when we see this, so it seems to leave no other explanation other than that portion of the case is unsupported when the brass expands there.

I had it happen early when I was first loading for 6.5 Grendel, but I just don’t see it and have not seen in personally over most of the last 15 years, across many different 6.5 Grendel AR-15s I ether own or help people with.