Suppressed .300 BLK Cycling Issue

[SNBI125]

Hello. I could really use some help diagnosing this issue. Here's an overview of this build:
80% AR-15 lower
Pro2A Tactical .300 BLK 7.5" upper w/ adjustable gas block
BCM pistol buffer kit w/ H3 (started w/ H2) carbine buffer

This ran fine unsuppressed w/ supersonic 150gr ammo, but my plan all along was to run it suppressed w/ subsonic. It runs ok w/ 220gr ammo, except that the bolt doesn't lock open on the last round, at least not consistently, and not every brand of ammo I've tried cycles very well.

The real issue is with 190 - 200gr subsonic ammo. I bought some defensive ammo in this range from Discreet Ballistics and would like to get it to cycle that.

With the H2 buffer I started with, the 190gr wasn't cycling at all. I switched to an H3 and it was better, but not great. The really strange issue was that it would cycle after the first round, but after firing the second round, the third one just stayed in the magazine (not getting jammed trying to feed or anything). That was with the 190gr ammo. I also have some 200gr but haven't had a chance to try that at the range yet.

I'm also wondering if the magazines could be making a difference. My understanding was that 5.56/.223 and .300 BLK magazines were essentially interchangeable, but when I was having those latest issues it was with a 5.56 magazine.

I've had the gas block basically all the way open (should I just remove the screw?).

Anyway, I'm wondering if an even heavier buffer would help. I found an H4 and will probably order that unless someone has a better suggestion to consider.

Thanks in advance for any advice.

 

[2ARay]

I'll give you a couple suggestions in my unprofessional opinion.

Maybe try an Odin Works adjustable buffer. Will give you several options on weights.

Also. check the diameter of the gas gas hole in barrel. I had one that I had to enlarge slightly. Made all the difference with adjustable gas block. You will have to search for the correct hole size. I used a proper diameter drill bit to measure. For some reason .093-.097 comes to mind but I not positive on that.

Make sure gas tube is correct length and engaging the proper length in the BCG gas key.

 

[no4mk1t]

Make sure the gas block is properly aligned with the gas port and is a snug fit and doesn't leak.
As mentioned by Ray, check the gas port for burrs and proper diameter.
Try a standard buffer. Once you get it to function with that, you can experiment with the various H buffers.
Make sure the BCG is adequately lubed. A new phosphate BCG in a new upper receiver make take a bit of shooting to wear in a little. Remember, with subs, you don't have the extra power that full power ammo has.
Use USGI or Mag-Pul 20 rnd. mags until it's reliable. A full 30 rounder puts a lot of drag on the BCG.

Once you get the bugs worked out and it cycles properly, adjust the gas block as follows:

Close the screw all the way.
Open it 1/2 turn.
Load a mag with one round. Fire and see if it locks open. (It won't at first)
Open the screw 1/2 turn at a time and fire one round until you get lock back.
Open another 1/2 turn to ensure functioning when dirty.

Suppressed AR's get dirty. Back pressure from the can means more gunk flows back thru the barrel into the upper. Clean and lube more often than when shooting un-suppressed.

And you'll want to mitigate the gas that leaks out around the charging handle and makes your eyes water. If you're a cheap bastard like me, you don't want to spend $50+ for a gas buster charging handle. You can DIY for about 25 cents worth of RTV.

DIY Gas Buster Charging Handle

Thought everyone has heard of this, but after talking with another member in a PM, maybe not. High Power shooters have been doing this since the 90's, so I thought everyone else was too. To keep the action gas from leaking out the back of the receiver and making your aiming eye water, there is...

www.patriotgunbuilders.com

 

[Mooner]

Can't remember who but I saw Gucci AR built that didn't cycle even with different buffer weights and adjustable gas block.

The solution was reduced power action spring.

May consider this in your diagnostic process.

 

[no4mk1t]

An important thing to remember is that if you want an AR to cycle subs, you really want pistol length gas. My first 300 BLK AR came with carbine gas and I didn't know any better at the time. This was before Remington standardized it and named it 300 BLK. I was making cases from LC 5.56 brass and trying different powders. The only powder that would make it cycle and stay subsonic was AA 1680.

 

[SNBI125]

Thanks for all the suggestions. There are definitely a few things I haven't tried yet. I've only put about 200 rounds through this (maybe half of those suppressed) but I can certainly make a point to clean it before I get it back to the range with whatever other change(s) make sense to try. I'm pretty sure I lubed it before I first used it but that was actually a while ago (I bought the upper about 18 months ago, fired it a few times with a different lower, then it was idle for a while until I built the lower a few months ago). I did just have another build fail to cycle because I had forgotten to lube it before the first trip to the range. Doh! It ran perfectly after oiling.

I bought this as a complete upper but I will check out the gas block alignment and hole sizes and make sure those are all correct. I did sort of eyeball the alignment at one point based on Pro2a Tactical's suggestion, and it looked right, but I'll take a closer look. And this is a pistol length gas system, according to the specs anyway.

The spring is a "carbine action spring" from BCM, from their pistol buffer kit. So the reduced power spring may be worth a try. The H3 did make a slight improvement over the H2, so I'm thinking that's in the right direction, but not convinced an H4 will necessarily fix everything.

 

[no4mk1t]

The H buffers are designed to add weight to the reciprocating mass for carbines that are very over gassed to begin with, to delay unlocking and slow down the cyclic rate. In your case, you probably want to reduce the reciprocating mass to get initial functioning sorted out. If it turns out your gas port was out of alignment and partially cutting off your gas, then you can experiment with buffers after it will run with the standard buffer. Always start from a state of proper functioning and only change one variable at a time.

One way to check port alignment.

 

[clm2112]

After you either increase the gas available to cycle the action, or reducing the force of the main spring, consider also reducing the mass of the bolt carrier and buffer. You are trying to find the spot where the two different conditions overlap. (i.e. gas volume vs reciprocating mass when running sub-sonic with a suppressor attached and supersonic without the suppressor.)

It is a tough nut to crack. I like to think of it like tuning the carb on a car.... finding the sweet spot of jet size, ignition advance, idle air, pump shot, etc. that allow it to operate over a wide range of conditions. It is never the most efficient at any condition, but it is usable across the board.

Just curious, but were you trying to run the subsonic ammunition without the suppressor on the end of the barrel?

Also, I don't know if you realize it or not, but you can remove the weights from the buffer. Drive out the roll pin, remove the teflon plug, and you'll see and remove the metal weight "pucks" inside. So instead of taking material off the bolt carrier, dump weight out of the buffer first and reduce the spring force until you get it into the range the adjustable gas block can control.

Last, the suggestion to start working with old 20 round M-16 magazines is sound. The shorter and straighter mags require less force to strip the rounds out than the 30 round magazines... so it is less likely the light bolt and spring gets hung up feeding a round.

 

[clm2112]

One other quick thought.... you don't need to use dedicated 300 blackout magazines. The round feeds fine in most cases out of a regular 5.56 magazine. If you look at magpul's dedicated 300 blackout magazine, what they have done is change the molded insides to account for the bigger bullet and moving the shoulder back on the case. What they are really trying to do is prevent the stack of rounds getting jammed up inside the body of the magazine. That is more about their polymer magazine than anything else (sort of a solution to a problem they created.) I haven't had an issue feeding the rounds out of the aluminum M16 magazines.

 

[SNBI125]

Just curious, but were you trying to run the subsonic ammunition without the suppressor on the end of the barrel?

I did, but only for the sake of only changing one thing at a time. I started off with supersonic and no suppressor, just to check function of the firearm with nothing added. As soon as I tried a few subsonic rounds without the suppressor, I realized it wouldn't cycle. Then added the suppressor with the 220gr subs, which were generally cycling much better than the 190-200s I'm dealing with now.

 

[clm2112]

I did, but only for the sake of only changing one thing at a time. I started off with supersonic and no suppressor, just to check function of the firearm with nothing added. As soon as I tried a few subsonic rounds without the suppressor, I realized it wouldn't cycle. Then added the suppressor with the 220gr subs, which were generally cycling much better than the 190-200s I'm dealing with now

Good, so the pistol's energy budget is right around the where the heavy subsonic rounds muzzle energy with a little help from the suppressor. Which means, you are probably pretty close to the maximum reciprocating mass needed to operate the pistol. Make the buffer lighter (reduce the mass) and/or lighten up the recoil spring.

If you never intend on shooting the pistol with supersonic rounds, you can lighten it a lot.

 

[Stephen G]

I have the Odin and the Strike Industries Optimus adjustable buffers. The Optimus is my favorite BY FAR. Super easy to adjust the weights.
I also prefer Aero precision gas block. My Odin works great, just a freaking nightmare to adjust with pistol length gas and 14" H/G and suppressor all in the way.
It helps if you can vid the ejection or have some1 stand back and watch.

In my case it turned out, actually couple times, it was OVER gassed/ too light of a buffer. Bolt was traveling too fast to strip the round from mag and/or allow bolt catch to rise up.

Completely shut, 1 click at a time, 1 round in mag at a time, till it locks open on empty mag.
Then I dick around with buffer weights for different weight rounds. IF THERES a issue.
I also run the wilson combat flat wire buffer spring as standard in all my ARs.

 

[SNBI125]

Thanks for all the advice. First steps, I removed the hanguard and got a closer look at the gas system. Everything seems to be aligned correctly and the end of the tube looks like it would meet right up with the BCG.
Next thing to try seems to be the buffer. The place I bought the upper from suggested going heavier, and switching from the H2 to the H3 seemed to be a small improvement, but most of you seem to think lighter is the way to go, and that seems to make sense. I'm going to order the SI Optimus - it looks like it has a much wider range of weights and it's about the same price as the Odin.

In my case it turned out, actually couple times, it was OVER gassed/ too light of a buffer. Bolt was traveling too fast to strip the round from mag and/or allow bolt catch to rise up.

I also have been assuming that I'd want the gas block all the way open but from this it sounds like that may not be the case. I will keep this in mind. For now I will clean and lube this upper before I put it back together. Hopefully I can pick up that buffer and get this thing back to the range some time in the next couple of weeks.

 

[SNBI125]

Wow, you guys are awesome. Huge improvement with this today at the range. Still need a few tweaks with the 190gr ammo, but the 200gr was running perfectly.

I loaded the Optimus buffer to the 3.0 oz. configuration (I was previously using 4.6 and 5.4 oz. buffers). I left the gas block where it was, just a couple turns from fully open. Started off with one round at a time of the 200gr, and it was locking open consistently from the first try. Started adding a couple of rounds and it cycled perfectly. From there, I closed the gas block all but one turn, and got it to cycle and lock open after about 4 more turns.

The 190gr was certainly much better than before, but not consistent. I started opening the gas block about 1/2 turn at at time. It would lock open 2-3 times, then it wouldn't. I started loading 2-3 rounds at a time and it did cycle well with maybe 1 failure to feed, but the LRBHO was probably about 50-70% overall. I opened the gas a little but at a time but after firing 36 rounds it never got to 100% locking open. I only had about 60 rounds of that ammo to begin with so I'll have to get some more before I try again. I waited a year to get the first batch - not sure whether they're still having supply issues or not.

Regardless, this was a huge improvement. Not sure whether I need to go even lighter with the buffer weights or add a bit more weight. Seems easy enough to swap out the weights, even at the range. After the 190s, I switched back to the 200gr and another 20 rounds cycled and locked open perfectly, and I was able to close the gas block a little more, but there was still enough coming back to be pretty irritating. Even the magazine was smoking when it was stitting on the table. I'd be happy if I can adjust the buffer or change the spring if that would let me close the gas block even more. Thanks again for all the advice.

 

[no4mk1t]

That's what the knowledge base here is for, for members to avail themselves of.
Glad you have found answers for your issues.

 

[SNBI125]

Hey all... finally getting back to this build after a few months, didn't realize it had been this long. Been busy troubleshooting other builds and getting around to buying more of this ammo. Anyway... I went back to the range with this today, after adjusting the buffer from 3.0 oz down to 2.6. Definitely another improvement. With 25 rounds (about 3-4 rounds per magazine), the 190gr (which is basically a practice version of the defensive ammo I plan to stage in this) cycled 100% with no failures but only locked open I think exactly 50% of the time. I then tried some of the 200gr and 220gr and both of those cycled and locked open 100% (about 35 rounds total).

The gas block was still most of the way open, so it seems like I may need to go even lighter with the buffer to get that LRBHO to 100% with that 190gr, so I'll probably try a bit lighter with the buffer on my next trip back. I just hope that doesn't cause any other issues - seems like maybe not, since this last adjustment did not seem to. I'd like to get it to cycling and locking open consistently then see if I can close that gas block a bit.

 

[Stephen G]

Its a dance, sometimes a mosh pit.
Keep a note book of your findings, it helps next year when you find a new ammo.
Most phones nowadays have EXCELLENT video capabilities and can be tripod mounted. This will save you 100000000 of rounds.

 

[SNBI125]

Yeah unfortunately the range I usually go to doesn't allow any photography (presumably for privacy of others at the range). Anyway, I went from 2.6g to 2.3g with the buffer yesterday, and at first I thought that did the trick. I was loading 2 rounds per magazine and the first 7 locked open with the 190gr ammo, but only ended up with 8 out of 10. An improvement but not perfect yet. I tried 10 rounds of my 200gr and that locked open all 5 times. I had run out of the Federal 220gr I shot last time, so I used some Remington 220gr which had previously given me multiple problems in this gun, including just getting the first round chambered and some cycling issues. Anyway, I didn't have those problems but that one only locked open on 3 of 8 magazines. So I'm not sure I can say that it's too light for 220gr but honestly I'll most likely use the 190 or 200 going forward so it's ok if that's the case. Planning to drop the buffer to 2.1g next time and see what happens.

 

[Bongo Lewi]

Hello. I could really use some help diagnosing this issue. Here's an overview of this build:
80% AR-15 lower
Pro2A Tactical .300 BLK 7.5" upper w/ adjustable gas block
BCM pistol buffer kit w/ H3 (started w/ H2) carbine buffer

This ran fine unsuppressed w/ supersonic 150gr ammo, but my plan all along was to run it suppressed w/ subsonic. It runs ok w/ 220gr ammo, except that the bolt doesn't lock open on the last round, at least not consistently, and not every brand of ammo I've tried cycles very well.

The real issue is with 190 - 200gr subsonic ammo. I bought some defensive ammo in this range from Discreet Ballistics and would like to get it to cycle that.

With the H2 buffer I started with, the 190gr wasn't cycling at all. I switched to an H3 and it was better, but not great. The really strange issue was that it would cycle after the first round, but after firing the second round, the third one just stayed in the magazine (not getting jammed trying to feed or anything). That was with the 190gr ammo. I also have some 200gr but haven't had a chance to try that at the range yet.

I'm also wondering if the magazines could be making a difference. My understanding was that 5.56/.223 and .300 BLK magazines were essentially interchangeable, but when I was having those latest issues it was with a 5.56 magazine.

I've had the gas block basically all the way open (should I just remove the screw?).

Anyway, I'm wondering if an even heavier buffer would help. I found an H4 and will probably order that unless someone has a better suggestion to consider.

Thanks in advance for any advice.

I struggled with getting a 300 BLK AR pistol build running reliably, both suppressed and not. Two issues that you have already zoomed in on. The buffer spring and the gas system.

I suppose that mags could be it, but there is no difference between 5.56 and 300 BLK mags. As long as you are using good-quality mags I don't think that's your issue.

It sounds like not enough gas to cycle the bolt. In short, more gas, and less spring.

Gas block may have become misaligned. or the block/tube may have a blockage. Carbon, shavings?

I'd also consider swapping the BCG if you have another AR pistol,

How long is your barrel? One of my discoveries was the 7.5-inch barrel I chose had an undersized gas port. You might have to open it up more or swap the barrel. I swapped my troublesome 7.5 to a 10-inch barrel and the problem disappeared. Then I compared the size of the gas port. Not the same. I opened up the 7.5 and put it back on. Voila.

As I'm sure you know, the only purpose of an adjustable block or carrier is to reduce gas flow when using a suppressor. It can be difficult to get it set correctly for a can. It's a trial-and-error kind of thing. Unfortunately, gas blocks don't make precise adjustments easy.

The bottom line is to get it running reliably unsuppressed. That should narrow the problem and point to the gas block not working as expected when a can is mounted.

Good luck!

 

[SNBI125]

Thanks for the response... I actually did start off running this unsuppressed, basically just to ensure it was working before introducing any other variables, i.e. the suppressor. I had no issues at all with at that point.

This is also a 7.5" barrel. I'll have to take a closer look at the gas port and make sure it's all as it's supposed to be. Once I got to a point with this where I was having cycling issues with the gas port completely open, and I turned to the buffer, I've tried not to touch the gas block. It's still mostly open (about 7 full turns from closed) so given that I'm still having a bit of an issue with the bolt not locking back every time, I'm still leaning toward going even lighter with the buffer (assuming I don't see any issue with the gas port). I'm still hoping I can get this cycling consistently with further buffer adjustments, and then see if I can close the gas port a few turns. It really blows a lot of fumes back in my face, even just a few rounds I have to turn around and get some fresh air.