I was at the range today with my M14's and Savage but the guy down the line had an AR in 300 blackout, new.

After talking with me and looking at my targets, he got the impression I was an expert and asked me to check out his rifle. First thing I noticed is that it was bone-dry. I broke out the CLP and lubed the thing.

I then loaded a 10 round PMAG and shot the rifle. It would either short stroke and re-chamber the fired brass, or eject a case and jam when feeding the next round. Most of the time, it just didn't eject.

I asked him to observe the bolt as I shot and he said it only went back half way and went back forward.

This was the only 300 blackout I've ever shot. I told him it was most likely under-gassed but I didn't have my borescope to check the gas port.

Is there a specific common problem with 300 BO's and short-stroking, or am I on the right track. We exchanged numbers and I might reach out to him later to see if he needs help.

Also, he was shooting Armscor ammo.

Tony.

What weight buffer was being used?

Was the ammo super- or subsonic? What barrel and gas system length? Does the gun have an adjustable gas block (that's too restricted)?

Probably unrelated to what you observed, but I'm assuming a standard 5.56 PMag and not a .300BLK specific one (since those are only available in 30 rnds at the moment). Heavy subsonic loads didn't always play well with 5.56 mag geometry and followers.

When I built my 300 BLK the advice going around was to use an adjustable gas block. I would suggest looking to see if this rifle has an adjustable gas block. If it does it probably needs to be removed and spend some time in a bucket of carb cleaner. Because the reason they aren't so popular today is because they tend to clog up.

300/221 gas systems can vary greatly in length and what they're set up for.

So can how the ammo is loaded.

He may have found an unhappy match, not enough gas for what he's got.

(of course it could also be run of the mill new AR not cycling troubles, misaligned ports etc)

Are you sure he had 300BLK ammo and didn't load it with 5.56? Because 5.56 would not seal in the barrel and thus would not get much gas pressure through the port.

Just saying...

I actually didn't notice the rifle was a 300 BO until I cleared a jam and noticed the round was a 300 and not a 223, so I know for sure he was using 300 BO ammo.

Tony.

If it used to function but now doesn't, I'd suspect the ammo. Lube helps

As mentioned, buffer weight, spring system, bullet weight, etc are all the usual suspects.

It was a brand new build. First time to the range.

I'd suggest checking the gas block. If home built could be misaligned. Either way the bolt is not getting enough gas to operate.

My rifle will shoot subs without a can, however I do not use an adjustable gas block. That isn't always the case though and could be leading to issues.

A lot of builders grab whatever weight buffer is available. Or monkey with weights trying to fix a simple gas issue.

Standard buffer weight for the 5.56 carbine is the H2…same one for the 300 BO. An H2 or H3 is the way to go with a 16” barrel and carbine length gas post.

https://www.80percentarms.com/...s-and-how-to-choose/

But first, I’d check that the gas key is locked tight and staked, then the front sight block which holds the gas block is also in the right position…there should be a small gap between them and the step on the barrel(the thickness of a paperclip), and make sure the block is lined up and not off kilter-viewing from the muzzle, the FSB should be clocked at 1200. Or any gas block for that matter.

This link at the bottom of the page shows a misaligned gas port/Gas block

https://www.arbuildjunkie.com/...5-gas-system-issues/

Then, you have to remember that there will be some gas sneaking out and around the gas block or FSB until 50 or so rounds have been fired-that carbon will effectively seal off any out of round barrel/gas block specs.

You are on the right path with oil. ARs need to be run wet. Squirt some oil on it and the gun will sling off he excess amount it doesn’t need. Bolt carrier group and bolt especially.

I have a 10” 300BO barrel and use it on a lower it shares with a 16” 5.56 barrel. But I used an adjustable gas block on the 300BO due to its length and the use of a can. The lower uses a standard H2 buffer and spring so that it also works with the parent upper in 5.56

Hammer spring installed correctly?

Lots of good advice here. Sorry to say my 16” build runs fine, but will not cycle heavy bullet subsonic unless I have the van on it.
Ammo is the number one place to start.
The hammer spring is also a good idea, even if factory built.