Home Forums Hunting & Shooting AR and Tactical Rifles Had a PCC hickup today...

Converted a damned handy PC AR pistol to fairly handy 16" PC carbine. Aero Epc9 receivers, Faxon skinny barrel. Thought all was well. Wife's hiking gun. Blasting away with 124 gr FMJs. Yah baby. She's going 100% hits....

Then put in a mag loaded with a handful of Underwood 147gr +P Hard Cast FN, item 719. Blam, blam, boom!
Somebitch, third one sounded fubar, mag flew out onto the ground. Cleared some busted up brass and luckily checked the bore. Luckily. Bullet lodged about 3" ahead of the chamber.

Fugg. I should have kept the brass chards but didn't. Pounded out the bullet using brass rods at home. Just a bullet.

Feeling pretty sure it was the cartridge. At least can't think of anything besides a dud. Going to take it back out and blast more...


Any thoughts on what else to check or test?

Added
Could it have fired before locked up?

Can't say for sure but ammo it sounds like.

Glad you were not hurt.

It's probably fine. May want to check the barrel extension for cracks.

Good idea, I'll tear it down.

I believe pistol rounds (9mm, 45, etc) are too low of pressure to crack the extension, so I'll leave it up to you.

No signs of issues... Cleaned some stuff up, torqued a bit more diligently and lubed better. I'll give it a whirl.

Such out of battery firings are not uncommon with PCCs, because there is technically no such thing as “lock up” with a blowback operating system. When firing residue accumulates to the point where complete cartridge chambering is prevented by it, much depends on the tolerance built into the the gun’s disconnector as to how far the cartridge can be out of battery and still be fired. Because your bullet didn’t get very far down the barrel, it sounds very much like most of the propellant gas escaped down the mag well instead of pushing the bullet, indicating the cartridge was pretty far out of the chamber when it fired and ruptured.

Besides frequent cleaning, another measure you can take to minimize the risk is to install the strongest recoil spring which will still allow minimum proper cycling. If the empties are being flung more than just a few feet, a stronger spring can be used. The stronger spring will tend to power the breechblock, and the cartridges it’s feeding, through more crud to completely chamber rounds.

Can the firing pin stick on those?

Don’t know much about ARs, but I’m dipping my toe in again with a .22mag. Might need a 9mm upper and am thinking the CMMG delayed blowback looks good, as it should for what they cost.

Blowbacks can fire out of battery. Get a bulged case or a very fouled chamber and interesting things can happen. It's a thing for sure.

Yah, shouldn't even call this an AR, nor use the term lockup. Dinking around with it, sloppy trigger technique might be responsible. Not dirty, however, it was fairly dry. Things to consider. Appreciate the response.

OK, what is happening to make it fire out of battery is bolt bounce. The round is chambered and the extractor is over the rim. The bolt then bounces off the face of the barrel backwards and pulls out the case and you fire.

This is what is happening. How to fix it? You have to have a buffer that has moveable weights inside like a normal ar15 buffer has. It acts like a dead blow and prevents the bolt bounce.

How I fixed mine was to take a 9oz solid buffer, bored it out and put in 1.5oz of lead shot. Then put the plastic bumper back on. I also turned the extra thick base back to resemble a regular buffer. It ended up being 6.5oz total weight. I have not had a single out of battery since doing this. I can bumpfire complete 32rnd mags without a hitch.

Maybe, maybe not.

Hmmmm.
Firing out of battery while the bolt is in mid bounce. One would need to be amazingly quick of finger to accomplish that, if not using some sorta extremely rapid firing mechanical aid.

The firing pin can not reach the primer till the extractor slips over the rim of the cartridge. It only slips over after the round has been fully chambered. So explain how it is going to fire out of battery if the firing pin can't touch the primer.

Go over to ar15.com and talk to the Mad Machinist. He builds these things for a living. He builds blow backs and gas operated DI PCC guns. I was thinking just like the rest of you thinking it was firing before the round was chambered. But it is not. It is firing after the round was chambered and bounced back.

With a bulged or misshaped case, there is enough resistance for the BCG to engage the cartridge without it being fully enclosed in the chamber. That becomes an out of battery ignition and interesting things will happen.

It's a thing.

Sigh. Hmmmm. The +P was not a good idea me thinks.

I run my 9mm for mine over a +P load. I am getting 1375fps out of a 136gr bullet in a 8" barrel. You need a combined weight of 22-24oz of the bolt and buffer to run a normal 9mm blowback AR15. If you run higher pressure ammo you need even higher weights. Go to Macon Armory and have a look at their buffers. It will cure what is happening.

The Mad Machinist is also the owner of Macon Armory.

Follow-up.

Installed a Kynshot RB5000HP hydraulic buffer with standard carbine buffer spring and it seems to work like a charm with the heavy Underwood loads as well as Buffalo Bore Outdoorsmans and three standard pressure rounds.

I buy into the bounce back.....

Bolt bounce is a real thing and most 9mm buffers are solid, meaning the weights don’t move back and forth, actually they were a sold block of steel.

DPMS made a good heavy buffer with weights that moved.

There’s some high speed/ slow motion film out there showing bolt bounce and even though it was mostly a problem on full auto, 9mm carbines were notorious until the buffers with reciprocating weights came around.

At one point in time, Xtreme was selling some garbage brass that had a ledge inside of it.

Unbeknownst to me, I shot about 5 rounds of it through my machinegun and apparently the brass separated in the chamber on round 4 and round 5 tried to chamber and fired.

The bullet from round 5 traveled THROUGH the separated brass from round 4 in the chamber and lodged about an inch into the barrel.

I still haven't forgiven xtreme bullets for that crap.