I built a 300 blackout upper. The gas hole in the barrel was fairly large, didn't measure it but about 1/4". It is a 16" barrel carbine version.

Ok, so I've fired it a few times. After 2-3 rounds, it doesn't eject the spent cartridge. I've tried 110 grain, 125 grain bullets and 208 grain bullets. I do not have a suppressor on it.

Now does it require a suppressor to have enough back pressure to operate the gas system?

Or do I trim the recoil spring back one or two loops at a time to weaken the recoil spring?

Or Is there a muzzle break short of a suppressor that will put enough back pressure to operate?

Or do I replace the gas block with an adjustable gas block?

Which would you do? Don't have a lot of money left to spend on this project and at this time, I can't afford a suppressor.

I would never do a 300/221 without an adjustable gas block due to the wide variety of loads out there. From slow subs to fast light super loads.

Although first thing I"d try is to make sure the gas block on the gun is indexed correctly so its not blocking part of that port.

I don't think the port on my 300 blackout barrel is 1/4 inch or close, but I stopped the build on that years ago when I realized I'd have action noise to deal with along with the suppressing noise and moved to a bolt gun for the subs.

I don't recall if my adj block is locked in place yet or not, if its not I'll measure tonight and post again...

Don't bother with an adjustable gas block on a gun that doesn't fully cycle; you don't need to restrict the gas any further.

Maximum effective gas port size is 1/8" (0.125"), yours won't be any bigger than that.

I'd guess you have a carbine length gas system on that barrel; should have gone with pistol length gas. If you do already have a 1/8" gas port (it's probably smaller than that), there's not much you can do other than making sure the gas block is aligned properly, tube is aligned in the block, and the gas rings are installed correctly on the bolt.

Did you use an H1 or H2 buffer, or extra power spring? If so, ditch those and go back to standard carbine parts.

I still wouldn't build one without adjustable, even after you get it working.

Alignment is often the key for the gas block, in fact about every time I've seen the issue on a 223 it goes back to the gas block partially blocking the port.

1/4 inch... it just dawned on me... a .224 bullet... Whats bore diameter... and a .250 hole. LOL.

Everything is lined up properly. I took it apart and redid it. It is a carbine length barrel, and all standard parts. However, I didn't measure the hole at the time, but I thought it was big. I've built two other 223's and never had this problem. I may check into an adjustable gas port. Makes sense. I wanted to deer hunt with this, and sometime in the future get a suppressor when I get the money.

I've seen two types of adjustable gas blocks. 0.750" and 0.96", which one? Mine is carbine length.

Again, an adjustable gas port won't help your problem. They restrict gas, your rifle already doesn't have enough. There is absolutely no value at all in an adjustable gas block on an undergassed rifle; it would be like putting a throttle restriction on your Honda because it doesn't have enough power.

Nothing wrong with using one, but fix the problem first.

You have to measure the diameter of your barrel at the gas block, that is the number you're asking about. Yours is probably .750".

Carbine length gas in 300 Blk is known to be more picky about cycling. Please measure your gas port and report back; if it's not a full .125" already you might be able to open it up a little more.

Are you using factory ammo, or handloads? If handloads, what are your loads?

It could be you're using the wrong powder, or the loads are too light. Even if they are correct, but the rifle is still under gassed, you could get it to run right by moving to a slower powder.

Both hand loads and factory loads. Both will only cycle two, sometimes 3 rounds. I loaded per Lyman book for the powder used. I used H-110 powder. Also used the carbine chart.

Ok on the .750". Wasn't thinking. You are right. All the adjustable gas block will do is restrict rather than open it up.

I have read that the Lyman reloading book is a little conservative. I am also loading light and will work up. However, the factory loads do the same thing. I've also changed magazines, both steel and Magpul. Same results. The gas block is pinned in place, so it can't really be off. Gas tube only pins a certain way also. It does cycle the first round, but after than no. I've never seen this before on any of my other AR's.

Swap out your BCG with a known good.

If you have .223 ARs, I would try one of your other bolt carrier groups, and then a different carbine lower.

What was your 110gr load? I load 110gr Hornady SPs seated to 1.93inches over 19.5gr of H110. They cycle fine in my 16inch, carbine gas 300BLK with standard carbine buffer and spring.

Whose barrel or did I miss that?

What exact loads are you using? The supers may well be just too light, and H110 is not ideal for subs. It'll work fine in a pistol gas system but probably not in carbine gas; you'll probably be restricted to using 1680 or slower powders for that.

Do swap your BCG with a known good one, as recommended above.

Funny part still is if I wanted supersonic I'd just run an X 39....

300/221 is for subs only... plus you have to run 1680, its not the quietest powder at all for subs and a can..

Only reason I continue to pursue my AR upper in 300/221 is for trailing wounded deer with the dog... for that with a can and follow up shots, the 1680 will likely be quiet enough..

Hmmm deer season is just around the corner, need to test some of my bullseye loads soon and put the finishing touches on the AR...

Rost, I like that you are referring to it as 300/221. Were guys converting their XP100’s to 300/221 back in the day? Or is 221 Fireball the parent cartridge that JD Jones used when he was working on the Whisper? Or AAC?

221 fireball is the parent cartridge of the 300 Whisper and 300 Blackout. Somewhere prior to the introduction of the 300 BO there were 300/221 die sets offered by Hornady and custom contender barrels in 300/221 by various makers. The 300 Whisper is a proprietary round hence manufacturers other than SSK using the 300/221 nomenclature so old JDJ wouldn't sue them.

I still call it what it was first time... old fashioned I suppose.... sue me. LOL.

Damn names like blackout, like an anemic pistol type round is going to flatten something like a "magnum" rifle round kind of irritates an old bastard like me.

They are so close to the same thing it doesn't matter, except the better bullet selection in .308". We've discussed that before though, everyone has their own opinion.

Thanks for the history lesson. Good stuff. I wrongly assumed that the 300BLK started as a shortened 223 since that is the way most make the cases. I knew of the 221 Fireball, but didn't know that it shared case head dimensions.

I agree with you on the Blackout name being a little .. trendy.

I picked up a 300BLK barrel and dies just for kicks and paper punching. I don't plan to shoot subs or get a suppressor, but who knows.

Nephews x39 has 308 barrel. Does ok with surplus, shoots around MOA with 125 ballistic tips. Best of both worlds. IMHO. YMMV

Edit to add, a buddy started his AR as a sub gun, and now runs supers through a can, 115 barnes X of some type IIRC. Accurate enough, but not nearly quiet enough for my tastes.

At the point we started some of this it was all reformed brass. Then you could buy reformed, but it was still that, and with the advent of made brass, it changes things a bit.

But surplus x39 ammo for blasting is something you won't find cheap in the 300/221 hence my thinking, thats for subs, the x39 is supersonic and for blasting.

I used 17.5 g of Winchester 296 powder for a 125 g Nosler ballistic tip. Still after 2-3 rounds, won't eject. This is close to max load. Should I go max load?

I've run two different .300 BO's - one a CMMG, and the other a Daniel. Both worked flawlessly with subs or Supers, although the CMMG was not terribly accurate - about 1.5 to 2 MOA with handloads.

I'm thinking there is some other issue with the gun in question, as it should work well with either load.

without looking at load data, just in general, if a round won't cycle going a bit more powder isn't goign to change it.

Burn rates can change it though.

One other thought... just a random thought... sometimes the ARs are not smooth until you get 100 round through them or so...

And while thinking... the upper is lubed? Oil and not grease right? They dno't like to run dry... grease is to heavy... mobil 1 30 weight or such... thin film wiped inside upper with a finger, and then on the bearing surfaces of the bolt carrier group...

You'll find a lot of guys loading that a little hotter, as high as 19gr sometimes; I typically use 18gr of H110/296. 17.5gr should cycle fine, you have something else going on with the gun.

What rost said, and check out the gas rings. Is the bolt really stiff in the carrier?

I"ve personally never seen rings cause an issue, though they should be able to.

The are simple to check and lube, make sure you do look at that.

I haven't been able to get to the range lately. However, I'm going to try some factory 208g cartridges. I did notice today that it is very dry, the bolt, bolt carrier, etc. So:

I'm going to lub it more before I shoot.

I'm going to change bolts and bolt carriers with my .223 rifle to see what happens.

I'm going to put a 223 upper on the lower to see if the recoil spring works ok.

That should narrow it down without spending any more money other than cartridges.

I've rebuilt it twice and can't see anything wrong. Like I said, never had this problem with 223 uppers.

I did see where H-110 and 296 max loads in Lyman book is 17.8. (I used 17.5). The Hodgdon online said use 17.5 of H-110 with a carbine length barrel.

It is a heavy barrel. I do remember measuring the diameter of the gas hole in the barrel and it was bigger than minimum recommended. It was fairly large, so that isn't the problem.

Would the gas block have to be specific to 300 BO or is the standard block ok? If the gas tubes are the same what would that matter? From everything I have read, only a barrel change is necessary from 223/556 to 300 BO.

The gas block's are all the same.

Here are loads that work in my S&W M&P15/.300BLK:

Barnes 110g TAC-TX/20.0g H110 (~2400fps per load data)
Hornady 110g V-MAX/20g H110 (~2382fps per Hodgdon)
Nosler 125g BT/20.7g AA1680 (~1962fps per Nosler)
Hornady 208g A-MAX/11.0g AA1680 (~1050fps per Hornady)

Sounds like you're on the right track for troubleshooting.

The gas block and tube are not specific to the 300, only the barrel.

What "minimum recommendation" for gas port size did you look at? Plenty of people (and some barrel makers) seem to think they can go by 5.56 specs on the gas system and port size dimensions - that doesn't work for the 300, and you end up with barrels that don't cycle, like yours.

I'd Try this before anything else if it were me.

Dry, one I had not shot in likely 20 years, got me a few weeks ago... I never even checked.. handed it to our God child and it didn't work right.... WTF, its a colt gun and you know those are worth every penny and work right ALL the time... way better than a damn RRA... LOL.

It was dry... I guess the lube went away over the years.... and probably some dust around too..

Lubed it with oil, I think I had some 5/20 synthetic in the mule... all was fine...

That could by my problem. I didn't lub it much to begin with. Someone on some website said not to overlub it. I guess that might help and help break it in.

I bought a Benelli SBE several years ago. Eveyone said it wouldn't cycle low bird shot loads and clay sports shots well and would jamb. Well I got a box of 3" mag buckshot and shot the whole box. Then I lubed it up real good. Now I shoot it at the clay range just fine. No jambs even on #8 shot 1 oz loads. And, I can still shoot 3-1/2" mags for geese or turkey.

Run it wet, and they'll go forever. Shouldn't be so wet that it gets oil on you, but definitely not dry. These things can be nasty dirty with caked up fouling, and still run fine as long as you keep them wet with oil.

No grease, like rost said.

I'd never break anything in dry.....

I use Breakfree CLP. Hope that is ok. Works on my 223's.

Some sort of actual gun oil would be better.

Breakfree CLP is what I've used for many years without any issues.

When in a pinch I use synthetic oil.

I"ve never had an issue with either. YMMV