I can’t provide a lot of details about this question (it’s not my gun), so I’m looking for the best guess/advice I can get. A short (~11") barrel AR type rifle chambered for 300 Blackout that I do not believe has an adjustable gas block does not cycle properly with some loads.

The gun fires and ejects the fired case properly, but the bolt does not cycle far enough to pick up and chamber a round from the magazine. The problem occurs with what I believe is a PSD 220 grain FMJ subsonic load, but not with Hornady Black 110 grain V-MAX ammunition that functions fine. The advice given about the problem was to install a heavier buffer (exact type I don’t know), and that was tried without curing the problem with the PSD ammunition.

I don’t have a lot of experience with the 300 BLK cartridge or with the type of short cycling malfunction being experienced, but it seems counterintuitive to use a heavier buffer when the problem is evidently insufficient BCG movement that prevents picking up the round from the magazine.

Comments?

Check the gas key for being staked properly, or loose
See if the hole in the barrel is too small-you may have to open it

Lotta possible culprits, but going to a heavier buffer doesn’t make sense.

Our patrol rifles started exhibiting the same issue, but only after getting somewhat dirty, say after 150 rounds or so. (Rifles were Colts and FNs in 5.56, using Federal 62 gr .223 ammo,various barrel lengths nlt 14.5”, various buffer weights and some equipped with Sakers).

After trying various solutions, I found that the end of the gas tube that fits into the carrier key had worn down enough to cause short-stroking in moderately dirty rifles. I also learned that there exists a gauge to check the gas tube diameter (it’s a go/no go type).

That said, my personal 300 Black runs subs and supers flawlessly with a fixed gas block and H2 buffer. Keep us posted!

Heavier buffer doesn't make sense unless what it's really doing is cycling too fast, and the bolt is outrunning the magazine, cycling faster than the next round can come up. Unlikely here.

But if it's only doing it with 220gr subsonics, there may be nothing "wrong". It would actually be pretty common.

Thanks for the comments. You've given me a few things to offer as advice to check.

quote:

The gun fires and ejects the fired case properly, but the bolt does not cycle far enough to pick up and chamber a round from the magazine. The problem occurs with what I believe is a PSD 220 grain FMJ subsonic load, but not with Hornady Black 110 grain V-MAX ammunition that functions fine. The advice given about the problem was to install a heavier buffer (exact type I don’t know), and that was tried without curing the problem with the PSD ammunition.

I have the same problem shooting 300 blk out with a 16 inch standard gas system on a Rock River.

I tried different weight buffer and nothing works over 200 grain ammo.

So I always use anything less than 200 grain ammo.

I have issues with a 10.5" 300blk that wasn't getting enough gas to cycle subsonic ammo. I started with an H2 buffer that didn't cycle, went to H1 and then down again to a carbine buffer and got it to work half the time. Next move was change from carbine buffer spring to a Spinco Yellow Spring (reduced power) that got me to 70%. I have the adjustable gas block opened all the way.

This barrel was made 10 years ago by Black Hole Weaponry and it was an expirement with C5 gas length, slightly longer gas tube than pistol length that is used most often on 300blk. I now understand why the C5 gas length never took off. I'm going to have to have the gas port drilled out to make it cycle reliably.

Another thing to look at, I missed maybe the most obvious question. Does it have a suppressor? Most 300blk won't run subsonics without a one. As Cas said, one of those cases where there isn't really anything wrong, if it won't run subs without a suppressor. They need the backpressure from a can to cycle properly. Some 300s don't like the new flowthru cans either, they need the higher back pressure from an old school traditional baffle can to run subs (and the old cans still sound good on subsonic).

What magazine is being used in said .300 BLK Rifle? If Polymer, is it .300 BLK specific? The OAL of many subsonic .300 BLK rounds is a tad long for use in a std. 5.56 Magazine, and the projectile can drag such that reliable feeding can be affected. This is one of the issues the Magpul .300 BLK Magazine was designed to overcome. Just something to check...

Thanks for the information about the suppressor. My guns function properly using subsonic ammunition without one and it didn’t occur to me that it might make a difference. The one I’m asking about doesn’t have a suppressor and that lack’s effect on gas pressure certainly makes sense. That possible issue is good to know.

Added: The issue of gas pressure prompted me to compare the reported chamber pressures of subsonic 300 BLK loads with others. According to what I found the CP of the PSD load I mentioned above is probably about 20,000 psi. That’s similar to that of the 45 ACP which is often considered to be a somewhat low-pressure cartridge.

The point about the magazines is also something good to be aware of. I let the owner use several 300 BLK-specific Lancer and Magpul mags, but that didn’t help. Also I didn’t mention it before, but I believe it is a relatively new gun and probably properly maintained, so those factors are unlikely to have been the problem.

Again, thanks for all the comments. This has been a good education for me about something I was not very familiar with.

This link will tell ya how big the gas hole needs to be in different length barrels for the 300BO.

https://tacticalmachining.com/...-gas-port-sizes.html

The very first thing I always look at on an AR are the gas rings on the bolt and make sure the gaps are not aligned. Be amazed at how often that is the problem. Something very few even think of checking.

quote:

Originally posted by fischtown7:
The very first thing I always look at on an AR are the gas rings on the bolt and make sure the gaps are not aligned. Be amazed at how often that is the problem. Something very few even think of checking.

This myth, wives-tale, webz-lore, gun-nut-tale needs to go the way of the dodo. The rings rotate independently, every time a bolt it cycled. This topic came up many years ago in an advanced carbine class at Rifles Only. Jacob Bynum -- owner & instructor -- had the course members do a few tests.

First, we placed the rings in a perfect gap non-alignment -- 120 degree spacing. Then we carefully inserted our bolts into the BCGs, then BCGs into the rifles, and then hand-cycled the actions a few times. Afterwards, we carefully pulled the bolts from the BCGs. Nobody had 120 degree spacing on their bolt rings -- the spacing was all over the place.

Next, we did the same as above, but this time starting out with all ring gaps in the same place. After hand cycling, the spacing was all over the place.

Then we repeated both the 120-degree spacing and the gap alignment setup, but fired a few rounds. Nobody had any cycling issues, and yet again the ring gap spacing changed for every person's rifle.

Finally, for a few us willing to do it, we removed one ring at a time and fired a few rounds. For those who tried this, all ARs cycled fine with only 2 rings. Then we went down to 1 ring. IIRC, one student's gun wouldn't cycle. Mine cycled fine with slow fire, but mis-cycled once with more rapid fire. The instructor's rifle ran fine with only 1 ring, but he stated that it might have issues like mine with rapid fire.

Gas ring gap location is just a myth. If an AR isn't firing and there is gap ring alignment (although this would be really hard to test), there are larger issues with the rifle.

quote:

Originally posted by fritz:
Gas ring gap location is just a myth.

That’s what we were told in a Colt-sponsored AR armorer course many years ago. The instructor said it had been a hot issue when he was in the Marine Corps, but it was not valid. What I have found is that regardless of how they were spaced initially, after the gun had been fired the gaps tend to line up automatically. I believe that’s because of tiny burrs at the edges of the gaps that tend to catch on each other and then hold the gaps aligned.

I stagger the gaps after cleaning just because, but I have no doubt that it doesn’t matter.

And thanks, fritz, for that experiment description; something I will definitely remember for future discussions.

Staggering the rings is what they taught us in basic training, and I have just always done it. I don't know if disassembling and looking at them helped or what. But I have done it on several occasions and problem disappeared. I have read the myth argument, and I agree they move all over the place. Maybe I knocked some dirt out or something else.

When I was a unit armorer I found a few M-16\M-4 rifles with two gas rings. Don't know where the third one went and they were running just fine before I tagged them for the small arms shop.

This message has been edited. Last edited by: SgtGold,

I have a Larue 11.5” 5.56 barreled rifle that seemed to behave well with a Spikes T2, similar to an H2 weight. I now have a suppressor on it and I began to suspect light ejection, due to the distance being a bit short, so I tried a Spikes T3. The T3 caused short stroking while suppressed even though that’s what I see recommended on a few web pages. I put a carbine weight buffer in and it ejects smartly at 2 o’clock out to about six feet. That’s makes me think I need an H buffer.
The buffer can make a big difference.

A heavier buffer is going the wrong direction with a rifle that is short stroking with subs. Lighter is what he needs assuming that all the above comments about gas key and gas port hole in the barrel check out good.

The idea of a heavier buffer made no sense to me, but I didn't want to answer my own question.

It seems obvious that however the gun is set up, the one particular subsonic load that has a muzzle energy and projectile momentum similar to a decent 45 ACP round is simply not powerful enough to cycle the bolt properly.

Without an adjustable gas block, what do we want? Something that functions okay with a wimpy toy load to make the suppressor owners grin, but then causes excessive bolt carrier speed with a round that is intended for serious purposes?

quote:

Originally posted by sigfreund:
The idea of a heavier buffer made no sense to me, but I didn't want to answer my own question.

It seems obvious that however the gun is set up, the one particular subsonic load that has a muzzle energy and projectile momentum similar to a decent 45 ACP round is simply not powerful enough to cycle the bolt properly.

Without an adjustable gas block, what do we want? Something that functions okay with a wimpy toy load to make the suppressor owners grin, but then causes excessive bolt carrier speed with a round that is intended for serious purposes?

Griffin Armament sells a toolless adjustable gas block with a variety of options for the restricter plate that can be selected to allow for proper operation with subs and supers, if he is willing to swap that part out.

If the barrel port is too small, that won't help.