# Diagnosing and solving an AR pistol problem...



## Kauboy (May 12, 2014)

I did that thing I never recommend anyone do.
I screwed with a good thing, and made it a bad thing.

As documented here, I built an AR pistol in .300AAC ("300 Blackout").
The thing ran great. Not a single hiccup during its entire first trip out. I loved it, had a blast shooting it, and the optic I slapped on it was already dead on at 50 yards. I've honestly never had a better experience with a new gun.

Took it to the range yesterday, and it was a whole different experience. Ejection failures, failures to feed, bolt wouldn't lock back on an empty mag.
I started my normal diagnosis... 
I used a different mag than last time. Check the mag. It's a friggin' PMAG. Surely that's not the problem. But it *IS* a 5.56 PMAG, and not a specific .300 mag. I checked the follower, loaded and unloaded it, looked for any feed lip problems. Nothing seemed wrong.
Did I use different ammo? Is this thing picky over ammo? Are the pressures different?
Nope, same exact Remington 220gr white box that I used last time.
A few mumbled curses and aggressive jam clears later, and I was fed up. I packed up and left. Thankfully, the day's earlier handgun shooting was a treat, so it wasn't a total loss. (plus, I got to hold my silencer before it's sent off to NFA jail, a tear may have been shed)

I got home that night and was wracking my brain to think of what changed. I know guys generally get adjustable gas blocks for short barreled rifles and pistols, and mine was fixed... but it ran flawlessly the first time out.
Then it hit me.
*"HEY STUPID, YOU ADDED A FOLDING STOCK ADAPTER SINCE LAST TIME."*

*GEEEEZZZ!!!!!!*

About two months back, I had the idea to make the pistol even smaller. Pack-able even. So I ordered a Law Tactical Folding Stock Adapter, and installed it.
For those unfamiliar, this adapter allows an AR's buffer tube and buffer attachments to be folded to the side when not in use. It makes for a great option when making a compact stowable gun. You just unfold the stock, charge the weapon, and you're ready to go.
But, there's a little thing I forgot about with this set up. To account for the added length of the adapter being installed between the receiver and the buffer tube, the packaging includes a spacer of sorts. A steel "button" that securely inserts into the back of the bolt carrier, and contacts the buffer when the whole thing is unfolded. This allows for a gap-less contact between bolt carrier and buffer.
What didn't occur to me until last night was, it's made of steel... and that ADDS WEIGHT!

All of the day's problems suddenly made sense. I'd increased the mass of the bolt carrier assembly. The buffer is stock, as is the buffer spring. They are tuned to a standard weighted bolt carrier group and normal round pressures. I'd just thrown that whole equation into the trash and thought "It'll be fine".
It. Was. Not. Fine.
Bad ejections? Yep.
Bad feed? Yep.
Failure to hold open? Yep.
Stupid fancy gadgets...

Now to seek advice.
Four things come to mind that will resolve this issue.

*Remove the folding adapter, return to stock configuration....*
:vs_laugh::vs_laugh::vs_laugh::vs_laugh::vs_laugh: No.
*Install an adjustable gas block, tune it to force more gas into the chamber to compensate for the heavier carrier assembly.*
Maybe, but I don't like this idea. Mainly because it inherently increases the filth returning to the chamber.
*Lighter buffer*
*Lower power buffer spring*

Which is recommended? What have others tried? Am I missing another possible cause?

Quick edit to include future use cases:
Keep in mind, I intend to run subsonics through this eventually when the silencer gets out of jail. What option best suits that scenario, and fixes my current problem? (please don't say gas block, please don't say gas block...)


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## Chiefster23 (Feb 5, 2016)

Gas block! It’s the only thing that will correct your present situation and then later be able to compensate for your new supressor.


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## SOCOM42 (Nov 9, 2012)

Whatever you have for gas now with a change to an adjustable block, remember,

Before you shoot with the suppressor dial back the gas and work it up til it functions properly.


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## Kauboy (May 12, 2014)

You may have guess that I had a feeling that was the only "right" way to do it. The others might be temp fixes, but I'm likely to see other issues once I introduce subs to the mix.


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## Kauboy (May 12, 2014)

SOCOM42 said:


> Whatever you have for gas now with a change to an adjustable block, remember,
> 
> Before you shoot with the suppressor dial back the gas and work it up til it functions properly.


Does "work it up" mean start closed and open a little at a time? Or start full open, and work it closed a little at a time?


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## SOCOM42 (Nov 9, 2012)

Kauboy said:


> Does "work it up" mean start closed and open a little at a time? Or start full open, and work it closed a little at a time?


From the closed position.

Suppressors create a lot of back pressure regardless of what they are on.

I worked with a california based company developing a can for the military.

It was for the sniper version of the M-14 eventually used in the gulf war.

We had to reduce the gas piston hole from .147" to .062" that is a hell of a drop.

With that short a tube on yours, you are way up the power curve and are going to need to cut way back.


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## Kauboy (May 12, 2014)

SOCOM42 said:


> From the closed position.
> 
> Suppressors create a lot of back pressure regardless of what they are on.
> 
> ...


Thanks for the clarification!
I'm not sure how far up I can be on the power curve, considering such a small amount of extra metal threw the whole thing out of whack like it did.
Remember, the .300AAC has a lower pressure than the 5.56, and less powder behind it.

I figured the whole thing would be "over gassed", like most 5.56 rifles, but that doesn't appear to be the case.
With a can on the front, that could change though. Putting a silencer on it could swing the pressure back to optimal and make the thing run like a top again. I just can't wait a year before I have a working gun again.

EDIT: Just found the spec on the "bolt carrier extension", and was surprised to learn it weighs 1.8 ounces by itself. That's a ~10-20% higher load added, depending on BCG weight, which I don't know for sure. Likely around 12oz.


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## Old SF Guy (Dec 15, 2013)

Kauboy said:


> I did that thing I never recommend anyone do.
> I screwed with a good thing, and made it a bad thing.
> 
> As documented here, I built an AR pistol in .300AAC ("300 Blackout").
> ...


See now?... You'd get along fine with my wife. Both of ya'll tell me problems that you don't need me to solve and then go and spoil the mystery by giving me the answer. Like a damn movie trailer showing ole Yeller getting shot..... that just ain't right is all...just ain't.....screw it....I'm going home.


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## RedLion (Sep 23, 2015)

I think that it is accurate to say that many folks that run suppressors will get an adjustable gas key versus an adjustable gas block. You may want to consider getting one versus that adjustable gas block. Brownells sells a Rubber City adjustable gas key for $52.

https://www.brownells.com/rifle-parts/bolt-parts/bolt-carrier-parts/bolt-carrier-keys/ar-15-m16-adjustable-gas-key-prod71557.aspx


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## Old SF Guy (Dec 15, 2013)

RedLion said:


> I think that it is accurate to say that many folks that run suppressors will get an adjustable gas key versus an adjustable gas block. You may want to consider getting one versus that adjustable gas block. Brownells sells a Rubber City adjustable gas key for $52.
> 
> https://www.brownells.com/rifle-parts/bolt-parts/bolt-carrier-parts/bolt-carrier-keys/ar-15-m16-adjustable-gas-key-prod71557.aspx


your just a suck ass....tell him what yu really feel......I'm hurt...by it all... feelings matter damn it!!!!!


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## csi-tech (Apr 13, 2013)

AR Pistols with suppressors are inherently over gassed. I would try a subsonic round. Worked well for our suppressed SBR rifles. very reliable after that. I think they were 65 gr. projectiles too. (5.56)


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## Kauboy (May 12, 2014)

csi-tech said:


> AR Pistols with suppressors are inherently over gassed. I would try a subsonic round. Worked well for our suppressed SBR rifles. very reliable after that. I think they were 65 gr. projectiles too. (5.56)


That might become my next problem, but my silencer is still out of my hands while I await approval.
At the moment, under gassed seems to be the culprit.


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## Kauboy (May 12, 2014)

RedLion said:


> I think that it is accurate to say that many folks that run suppressors will get an adjustable gas key versus an adjustable gas block. You may want to consider getting one versus that adjustable gas block. Brownells sells a Rubber City adjustable gas key for $52.
> 
> https://www.brownells.com/rifle-parts/bolt-parts/bolt-carrier-parts/bolt-carrier-keys/ar-15-m16-adjustable-gas-key-prod71557.aspx


Description on that seems to apply to a gun with my reverse problem.


> "enhances performance of low mass and standard mass GI systems"


I went and added mass to my standard BCG, so mine would be "high mass"(if that's a phrase), and would need a way of allowing more gas to the key.

That actually makes me wonder if an adjustable gas block would even solve my current problem.
If the gas port in the barrel is a fixed size, and we assume my gas block is full open already, how can I get any more pressure than I'm already getting? (I need to take off my block and see if it has the same diameter hole as my gas port)
If they end up being the same, an adjustable block won't solve anything, will it?
Bear with me, as this is new, but does it work like water pressure? If you decrease the pipe size, you increase the pressure? Or does it restrict the flow, and thus cut down the pressure?
Geez that sounds like a dumb question... "does gas pressure work like water pressure". I feel like high school me sitting in physics class would kick my ass for asking that question.

If the system is already "wide open throttle", then an adjustable block might not solve my issue. Until I get a party can on the front of it, and get that pressure back up, should I focus on reducing the mass of the buffer? It's a cheaper solution, and a damn simple swap.


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## SOCOM42 (Nov 9, 2012)

Kauboy said:


> Description on that seems to apply to a gun with my reverse problem.
> 
> I went and added mass to my standard BCG, so mine would be "high mass"(if that's a phrase), and would need a way of allowing more gas to the key.
> 
> ...


Either hole, block or tube can be oversize, the regulator controls the flow such as with a needle valve.


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## Kauboy (May 12, 2014)

SOCOM42 said:


> Either hole, block or tube can be oversize, the regulator controls the flow such as with a needle valve.


Right, so the stock system as it is now, if either hole is bigger than the other, I'm already getting as much gas to the chamber as I can, right? The needle valve comparison makes me think I could only lower the pressure by lowering the flow, which would result in a worse cycling of the action.

I suppose if the gas port was larger, and I got a gas block that matched or was even larger than it, I could increase the flow, but not the other way around. I'm not drilling my gas port, that's for sure.
I need to take the thing apart...
Gimmie a minute.


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## Kauboy (May 12, 2014)

Ok, the gas port on the barrel comes in at 2.66mm. The hole on the gas block was visually larger than the gas port.
So, the limiting factor for flow is the barrel's gas port.
Will a gas block still fix my issue? If so, how?
Be as technical as you wish, cuz I'm still not understanding how reducing the hole size will increase the pressure to a sufficient level to push back the ~2 additional ounces of weight I've added to the BCG+buffer mass.


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## Kauboy (May 12, 2014)

I'm starting to think the buffer is my fix after watching this video. (the only one I've found that actually covers my exact problem...)
Jump to 7:32





EDIT: Found an article stating that standard carbine buffers contain 3 steel cylinders and 3 rubber pads, all accounting for 1.9oz of the buffer's overall 3oz weight.


> If you take apart a standard carbine buffer (which requires only a punch and a hammer: simply drive out the retaining pin and pop off the synthetic pad), you'll find three steel weights and three rubber pads inside. If you weigh a standard buffer, you'll find it's three ounces; the steel weights and the pads account for about 1.9 ounces of that.
> 
> Carbine buffers are available in a variety of weights. Colt developed heavier models for certain applications, including the H1 (3.8 ounces), H2 (4.6 ounces) and H3 (5.4 ounces). The H1 uses two steel weights and one tungsten weight, the H2 uses two tungsten and one steel weight while the H3 uses three tungsten weights. If you have a standard buffer and you buy an H3 buffer you can use their weights to build both an H1 and an H2.


(source: https://www.rifleshootermag.com/editorial/changing-buffer-weight-on-ar15s/83752)

I took apart my buffer, and have those 3 steel weights and 3 rubber pads, so it's a standard carbine buffer.
With this extender being added to my buffer's weight (1.8oz + 3oz), I've basically stepped up to just over an H2 buffer's weight(4.6oz).
That's drastic.

Looks like a lighter buffer is in my future.
When I finally get my can, I can look at adding my standard buffer back to see if things return to normal, or add an adjustable gas block to allow for different loads.


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## Kauboy (May 12, 2014)

You know...

It occurred to me just now that I was shooting 220gr. rounds that have an average muzzle velocity of 1,015fps. That's subsonic.
I bet if I just stick with supers until I get my can, the darn thing will run fine.
Range testing to commence.

What a terrible and fun hobby.
:roll:


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## Kauboy (May 12, 2014)

The same company that makes my pistol brace apparently also makes a lightweight "extension" that is an exact replica of the 1.8oz one I'm having fits over, and it only weighs .64oz.

For $20, I'll try it first. If it solves my problem, it's a quick 10 second swap between the standard weight and the lightweight when I want to use different ammo.
Just have to fold the stock, push a button and pull the extension out, insert the other, and unfold.
If it doesn't work, I can build a lightweight buffer for $16 from the same company with .22oz aluminum weights. I was tempted to just get both but restrained myself.


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## 13JFO (Jun 2, 2020)

Coming from someone who literally just built a 300 BLK pistol with the intent of throwing a form 1 can on the end, get the adjustable block, it's the only way to not "cheat". I've been there myself. Tried to screw with buffer weights and springs, too finnicky and unreliable. Wasted money. Throwing an adjustable block on is simple, too.


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## Kauboy (May 12, 2014)

13JFO said:


> Coming from someone who literally just built a 300 BLK pistol with the intent of throwing a form 1 can on the end, get the adjustable block, it's the only way to not "cheat". I've been there myself. Tried to screw with buffer weights and springs, too finnicky and unreliable. Wasted money. Throwing an adjustable block on is simple, too.


Can you answer my question posed a little above with respect to an adjustable gas block?
I'll copy it here:


> Ok, the gas port on the barrel comes in at 2.66mm. The hole on the gas block was visually larger than the gas port.
> So, the limiting factor for flow is the barrel's gas port.
> Will a gas block still fix my issue? If so, how?
> Be as technical as you wish, cuz I'm still not understanding how reducing the hole size will increase the pressure to a sufficient level to push back the ~2 additional ounces of weight I've added to the BCG+buffer mass.


How will an adjustable help this scenario?


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## Kauboy (May 12, 2014)

The lightweight extension I ordered arrived Monday. I picked up a small scale to really get an understanding of the difference in the stock extension and this new one.
Pics below.
The black one is the stock steel extension. The brown on is the lightweight aluminum replacement.

(*EDIT: Click the pictures and run through them in the slideshow view for correct orientation. No idea why they are flipped in this view.*)
Both are exact in all dimensions except for weight.








The stock steel extension actually weighs more than stated online. (stated as 1.8oz)








The lightweight one is 70% lighter than the stock one with the real weight.








I'll try to get to the range and see if the issue has been resolved.
If/When my can shows up, this will likely no longer be needed. But that's so far away...


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## dsdmmat (Nov 9, 2012)

If you are dead set against opening the gas port, I suggest throwing some aluminum weights in your buffer. You can get them from KAK industries or pm me your address and I can send you three of them.


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## Kauboy (May 12, 2014)

dsdmmat said:


> If you are dead set against opening the gas port, I suggest throwing some aluminum weights in your buffer. You can get them from KAK industries or pm me your address and I can send you three of them.


Somehow I missed this reply. Sorry about that.
I hit the range again today with the lightweight extension installed. Cycling improved, but not to a trustworthy level. I am indeed going to be reducing my buffer weight. That additional .638oz, even though tiny, still just isn't low enough to allow a strong bolt cycle. I figure it's a combination of the pistol length gas system and possibly the gas port being slightly undersized. I'm just not comfortable with resizing the port, and don't know a place around here that I trust to do it for me.

I saw the KAK buffer parts when I was ordering this lightweight extension. That's what I'm going to go with. I truly appreciate your offer, but I'm going to build a whole new buffer for it with their parts instead. I want a solution that can be swapped in and out if needed without a roll pin punch. $26 for all parts and shipping, including 2 steel and 3 aluminum weights so I can tweak it to exactly the combination that works best.

Again, thank you for the offer though. :vs_cool:


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## RedLion (Sep 23, 2015)

I would suggest to strongly consider getting the gas port enlarged before you end up spending enough $ that you could have covered buying an entirely new barrel. Any competent Gun Smith that has worked on AR's can easily enlarge the gas port.


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## 13JFO (Jun 2, 2020)

RedLion said:


> I would suggest to strongly consider getting the gas port enlarged before you end up spending enough $ that you could have covered buying an entirely new barrel. Any competent Gun Smith that has worked on AR's can easily enlarge the gas port.


I second this. It's pretty easy to open up the port yourself even, if you have a good drill press or mini-mill. Compensating for the port issue with weights and such is probably going to be more expensive and still less desirable. Better to open the port, get an adjustable, and enjoy knowing it'll be reliable and properly "tune-able".


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## Kauboy (May 12, 2014)

So, "the algorithm" delivered another video to my YT feed that addressed this.

It turns out, short-barrel .300 manufacturers have been INTENTIONALLY under-sizing the gas port with the expectation that their barrels will be used with silencers.
Supposedly, with a silencer on the end, sufficient back pressure is produced to cycle and lock back a bolt when firing lower powered sub-sonic ammunition.
That's my end goal. Once I get my can, it will only ever be fired suppressed.

The whole reason I ran into this issue is due to a lack of ammo availability where I am, and a range restriction at the place I like to go.
My range does not allow FMJ rifle ammo of any kind. So, I have to use soft point or hollows. The only option I was able to find at my local stores was 220gr subsonic hollow points.

What it all boils down to is, do I need to use subsonic ammunition in an non-suppressed gun? No, but it's all I can find to use at the range for now.
When I took the gun out to my BIL's place, I brought boxes of supers, and the thing ran like a champ.
With the lighter buffer, it would cycle perfectly with subs, but still would not lock back.

So, I'm going to proceed with leaving the gas port as-is, shooting supers most of the time, and when I visit the range and can only find subs, I'll just deal with the fact that the bolt won't lock open.
A buddy of mine said I could compromise on the issue and take my FMJs to a bench grinder, nip off the tip of the jacket, and make them soft points. I'm considering this, since I've got lots of FMJ supers. Anyone done this? Any shape concerns for avoiding feeding issues?


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## Elvis (Jun 22, 2018)

On my 12" long barrel .300 blk the upper manufacturer put the gas port at the end of the barrel. Basically a carbine length gas tube on a pistol length barrel. With the bullet escaping the barrel just as the gas started going down the tube to push the bolt bolt cycling was unreliable, even with full powered loads. 
I took the AR pistol to a local gunsmith who explained it to me and moved the gas block 2" further back on the barrel giving the gas time to travel down the tube before the bullet exited the barrel. That fixed the problem.

I still could hear the Mil-Spec buffer spring rattle when firing suppressed so I got a JP buffer spring. https://www.jprifles.com/1.4.7.2_os.php I really like the ability to easily tune the JP buffer spring.

I load 200 gn Hornady bullets on 10.7 grains of H-110 loaded to a length 2.240" and a mild crimp to get 1060 fps. This is a very accurate load in my rifle that usually shoots sub MOA 5 shot groups when I've got a scope mounted so I can see well enough to aim well. Normally I keep a red dot on it. 
The rifle is now 100% reliable suppressed or not. I haven't had a FTF in over 2 years. @Kauboy


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