AR Tuning question

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brutus1776

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Jun 12, 2007
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#1
I have a BCM 14.5 rifle with stock bcm buffer system. I shot out the stock barrel, rebarreled with Gieselle 14.5 chf. Superlative adjustable gas. Stock bolt. I ran it in a class for 1500 rounds. No problems. I put on an sjc titan brake. The rifle runs but will not lock back on the last round. Even with full gas. When you hold bolt stop down on last round, it locks back on the carrier not bolt face. I put in a jp scs with stock spring out of another rifle and it works with full gas and locks back, but move that gas 1 click and doesn’t lock back.

What do I need to change on this? Lighter spring? Carrier? Buffer weight? Thanks!
 
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#2
To me that sounds like buffer weight. A lighter spring will do the same, but it’s going to slingshot the existing buffer into the back of the extension hard as cause a screwy recoil impulse.

That’s just my experience with them.
 
#3
brutus1776 said:
I have a BCM 14.5 rifle with stock bcm buffer system. I shot out the stock barrel, rebarreled with Gieselle 14.5 chf. Superlative adjustable gas. Stock bolt. I ran it in a class for 1500 rounds. No problems. I put on an sjc titan brake. The rifle runs but will not lock back on the last round. Even with full gas. When you hold bolt stop down on last round, it locks back on the carrier not bolt face. I put in a jp scs with stock spring out of another rifle and it works with full gas and locks back, but move that gas 1 click and doesn’t lock back.

What do I need to change on this? Lighter spring? Carrier? Buffer weight? Thanks!
Click to expand...
The smart ass response that popped into my head was to say get rid of the brake.

There are a couple of things you can do. First, if you're running the setup 1500 rounds at a time, I definitely wouldn't go lighter on the spring. That will probably cause you to have failures to go into battery when it gets dirty. This is, of course, assuming that you're using what would be considered the standard rate spring. If you're using a heavier spring, then drop down to whatever would be considered stock.

You could get rid of the adjustable gas block. All AGBs leak at least a bit. You could also reduce the buffer weight.

I'd get rid of the adjustable gas block and tune with buffer. weight.
 
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#4
brutus1776 said:
I have a BCM 14.5 rifle with stock bcm buffer system. I shot out the stock barrel, rebarreled with Gieselle 14.5 chf. Superlative adjustable gas. Stock bolt. I ran it in a class for 1500 rounds. No problems. I put on an sjc titan brake. The rifle runs but will not lock back on the last round. Even with full gas. When you hold bolt stop down on last round, it locks back on the carrier not bolt face. I put in a jp scs with stock spring out of another rifle and it works with full gas and locks back, but move that gas 1 click and doesn’t lock back.

What do I need to change on this? Lighter spring? Carrier? Buffer weight? Thanks!
Click to expand...
That’s interesting. Ive never heard of a brake causing less gas to go back causing short stroking. Does your buffer weight have any tungsten weights? If so Id swap them out for steel ones and see if that helps.

I agree with Tony on getting rid of the adjustable gas block. I would also verify port alignment with a borescope if possible when putting the new gas block on.
 
#6
Take the brake off and see if it returns to normal function.

If it doesn't, your gas block could have come lose or moved, gas tube could be leaking, gas key could be leaking, etc etc. I see the blocks lined up but that doesn't mean something else didnt change.

How did you install the brake, essentially I am asking what tools?
 
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#7
Yes, I did that Saturday when I was trying to tune this thing. The bcm device runs fine but the sjc has the “no lock back on last round.”

I did all barrel work with the Geisselle action rod and snap on wrenches.

The rifle runs like a sewing machine with or without Brake but no lock back on last round
 
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#8
WeR0206 said:
That’s interesting. Ive never heard of a brake causing less gas to go back causing short stroking. Does your buffer weight have any tungsten weights? If so Id swap them out for steel ones and see if that helps.

I agree with Tony on getting rid of the adjustable gas block. I would also verify port alignment with a borescope if possible when putting the new gas block on.
Click to expand...
I haven’t weighed the buffer it’s self but it’s whatever comes in a stock bcm lower.
 
#9
Did you try less gas or more? Try one or two click less gas if you have not.

I am assuming there is little to no recoil, but do you feel ANY bolt bounce? You could test other springs, but if its BCM is likely a standard spring and regular H buffer. Id doesnt seem realistic to go lighter.

Can you test another brand of brake? I have seen this same thing happen on an AR10 in 260.
 
#10
brutus1776 said:
Thanks guys. I had a buddy verify gas port alignment with a scope. It’s dead nuts. Might mess with the weights on an SCS before pulling the gas block/rifle apart. I’ll report back soon.
Click to expand...
FWIW, I "had" been running an SCS15-H2 on my 300 Blackout with a 10.5" barrel with AC591 brake + OCL Hydrogen L 7.62 can. I built a 2nd upper with an 8.2" barrel and it would cycle properly with SOME of my ammo (200gr and 220gr), it would not cycle with the 190gr or 194gr ammo. I replaced ONE of the tungsten weights with a steel weight (making it a H1 vice an H2) and now that 8.2" 300 Blackout runs ALL my subsonic ammo AND supersonic ammo with no issues!!! (Also running a Sup Arms AGB, running wide open. Was 1 click past [19 clicks open] wide open and was hinky every once in a while.
 
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#11
mettee said:
Did you try less gas or more? Try one or two click less gas if you have not.

I am assuming there is little to no recoil, but do you feel ANY bolt bounce? You could test other springs, but if its BCM is likely a standard spring and regular H buffer. Id doesnt seem realistic to go lighter.

Can you test another brand of brake? I have seen this same thing happen on an AR10 in 260.
Click to expand...

Yes, I went through the entire gas block adjustments 2 clicks at a time with 1 round magazines. Finally with the stock spring on jp scs and weights, I got lock back. With pretty much full gas. I turned gas 2 clicks either way and no more lock back.

Yes. Little recoil. Dot will hold the upper half of body A zone at 50 yards with proper mount. I’m unsure on bolt bounce, I’m not sure what to look for on that.

I may try a different brake at some point but hate to put this on shelf after I just spent 150$ on it to get a gamer carbine… fwiw. But it may come to that.
 
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#12
Take weight from the buffer or try a lighter spring or just clip a few coils off and test as you go. Just make sure the bolt locks back on the last round and brass isn’t ejecting towards the muzzle. There’s not a lot to this other than dwell time but that’s not your problem with that barrel and gas length you have. So in your case your just balancing buffer and spring weight with the gas.
 
#13
Under this scenario:

Bcm h buffer and standard spring, gas wide open, any other muzzle device or no muzzle device, the gun runs.

As soon as you swap to the sjc it wont lock back?

Does the ejection change when the brake is on and off? What I'm looking for is a change in bolt speed, trying to see if that's the issue.

I would also try a regular gas block with and without the brake. It really seems like you are on the ragged edge of the tuning window.
 
#14
10-4.
Thanks everyone.
I can’t remember for sure but I had the gas turned down from say 18 clicks on the SA arms block (full open) to around 25 (bleed off) or so for the class. (Xray alpha/Stoeger)

Stock bcm buffer components.

It ran like a Swiss watch. It was ejecting between 3 and 4. This was with the bcm “comp” that I had on upper from bcm. (It’s a fancy flash hider really.)

Threw the sjc comp on, and just started shooting. Ran like a clock, noticed no lock back on the 3 mags I shot. So I went to 1 rd mags to tune. No lock back with any gas combo. Held bolt release down on last one round mag I shot and got lock on carrier face not bolt face

Got home, put in scs out of other rifle and full gas. Got lock back. 2 clicks either direction no lock. I actually didn’t pay much attention to ejection pattern a lot at this point and switching between muzzle devices as I was lasered onto getting it to lock back.

Will try swapping devices and looking at pattern this weekend.

I have a sprinco reduced spring coming so I can get back to training (hopefully) this weekend with this gun. Then hope to tune the buffer weight on next months pew budget.

Thanks for all the adv. Could going to a low mass carrier be a silver bullet without have to tinker with buffer weights and springs and scs’s ?
 
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#15
If you change the weight in the carrier or buffer it will absolutely change how it cycles and will probably fix your problem but I’d still check a few things like.
When’s the last time you checked your gas rings? All there? Also Rings Shouldn’t be loosely goosey in the carrier but have ring tension when pushing the bolt head back and forth.
Carrier key is not loose?
Honestly never used a Bleed Off gas block but if it’s bleeding off gas, that’s not what you need right now.
 
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#16
If you end up removing the gas block, check the port diameter to make sure your barrel didn't slip though QC with an undersized port. It should be 0.076". If you are using a used BCG, try a new one or check things like the gas rings and gas key fouling, same with the gas tube. Also check barrel clocking or other issues causing mechanical drag. I assume that you're dialed in on the SA AGB being at 4.5 rotations CCW for full open.
 
#17
Does it not lock back on all your magazines?
20 Rd mags or 30? What brand ?

A good friend AR was doing just this
He wouldn't believe this old Jarhead knew ARs
You might try there before messing with anything else
 
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#18
The Knuckle said:
If you change the weight in the carrier or buffer it will absolutely change how it cycles and will probably fix your problem but I’d still check a few things like.
When’s the last time you checked your gas rings? All there? Also Rings Shouldn’t be loosely goosey in the carrier but have ring tension when pushing the bolt head back and forth.
Carrier key is not loose?
Honestly never used a Bleed Off gas block but if it’s bleeding off gas, that’s not what you need right now.
Click to expand...
I guess I should check the rings, but I changed the bolt out to a new bcm with the new barrel. There’s less than 2000 rounds on this barrel and bolt.

Per magazines, I’ve used about 10 of my training pmags in the class and a couple GI and the same pmags last weekend tuning. They lock back on all my other rifles and lock back on this one without the brake.

Yall have also about talked me out of the adjustable gas as well.
 
#19
brutus1776 said:
I guess I should check the rings, but I changed the bolt out to a new bcm with the new barrel. There’s less than 2000 rounds on this barrel and bolt.

Per magazines, I’ve used about 10 of my training pmags in the class and a couple GI and the same pmags last weekend tuning. They lock back on all my other rifles and lock back on this one without the brake.

Yall have also about talked me out of the adjustable gas as well.
Click to expand...
If simply adding a break is causing it to be slightly undergassed then you’re on the bleeding edge of being able to cycle and yeah you def wouldn’t want an adjustable gas block. Usually a break alone wouldn’t be enough by itself to cause short stroking etc..

If you haven’t done so already, I would also definitely check that you don’t have any loose gas key screws on the bolt carrier (set torque wrench to about 30 inch pounds and try to turn counterclockwise). I had a fairly new geissele BCG with maybe only 1500 rounds and one of the gas keys screws somehow sheared off and was only being held in by the staking, and I was racking my brain trying to figure out why I suddenly started short shopping and that was the cause. To their credit they responded within 24 hours to my warranty claim and replaced it without any questions asked and even wanted me to send the old one back so they could examine it.

www.snipershide.com

Heads up, make sure to have a spare gas key, screws, and sealant on hand if getting short stroking etc.

and preferably also a carrier key staking tool and complete BCG. I posted this story in one of the 5.56 reloading threads but it bears repeating here. I have a 5.56 gas rifle and it has a 14.5 inch super duty mid-length upper which shot fine for several thousand rounds with h2 buffer + super 42...
www.snipershide.com www.snipershide.com
 
#20
Worn out carrier will let gas bleed off and not let it stay in the carrier and bolt to pressurize the system. Or a out of spec carrier will do it too. When I need to change bolts, I change the whole BCG. Could also be as simple as the carrier key not sealing as well as it should bleeding gas.

There are a lot more causes of a bcg not locking back than just buffer weight and springs. Heck, even the hammer springs play a roll in how the bcg works. If it is a new spring it holds just a hair longer as the bcg needs to overcome the spring rate the hammer has re cocking it. There is a lot of stuff that works all together as a system to make a ar run. Some things you can get away with and all it will do is wear out other parts. When you have multiple things out of spec you need to find out what it is and fix it. but you also need to know how things work first. And most people don't have a clue how things actually work other than what is regurgitated on most forums.

Bring another bcg with you next range trip and go from there. Then go to a standard weight carbine buffer. Not a H, H1,H2......

Also you need to pull the gas block and see what size of port it has. You will need a good set of pin gauges to do this. You should have at min 0.068" for carbine to be reliable. And 0.076" for a mid length gas.
 
#23
Going back to the original post, it sounds to me as if the new barrel has a smaller gas port than the original. You probably still have the original barrel and it sounds as if you'll probably be tearing down to the gas port anyway. Measure and compare the 2 gas ports and perhaps you'll find the root of the problem. JMO
 
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#24
Right on. I did have a lock back prob after a couple thousand rounds on an spr with bartlein barrel with the same SA gas block system. I swapped the SOLGW bcg for another and and it’s run fine ever since. I assume the gas key was leaking just enough to screw up the gas. That bolt has worked in another full gas/duty style gun ever since as well.

I’ll try swapping bcg in this gun as well.

The gas tube is new with the block and bcg upon barrel replacement. The bcm stock barrel was key holing after about 35k.

I didn’t realize there was a standard buffer under an H. Lots of stuff to recheck here, thanks again everyone.
 
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#25
Update:
Couldn’t take it. Pulled buffer weight. It’s an h1. I disassembled and took out one weight and put in a wooden dowel.

First 1 round mag I got lock back, which I previously only got on the jp scs from one of my other rifles. I went bleed through mode about 3 clicks less gas, no more lock back. I went about 3-4 clicks below open into restrictive and I got no lock back.

I’m not sure exactly where it’s sitting right now as far as bleed off mode/full gas/restrictive. Plan to try it this weekend, possibly with reduced spring when it comes.
 
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#26
The plot thickens.

Shot it today. Had lock back on first 2 mags. Stopped getting lock back. Finally opened the gas all the way. Was getting lock back on 1 round mags. Switched to 2 round mags and shot doubles. Lock back roughly every other mag.

Took a bcg out of another rifle. Noticeably more gas/recoil. Lock back achieved. Was able to get 8 clicks or so on gas reduction before failure to lock. I gave it four clicks more open and ran it for 3 mags with 6-8 rounds each. Lock back and rifle recoils great and repeatable.

Plan is to buy a jp bcg this week. Hopefully just put the weight back in the buffer that I took out and this is done.

Is it a thing that bcm bcg’s leak on tuned gas system rifles but run great in full gas guns? My spr had this same problem of weird gassing and after swapping the bcg, ran like a clock ever since.

Should I attempt to warranty these bcm bcgs or try to seal the gas keys? Or move on with jp or something better??
 
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#28
I think the SA gas blocks operate better within the initial 18 clicks not in bleed off. My guess is you need to be between 12-18 clicks out.

Interesting info on the bcg. Either the gas rings are not sealing, or the gas key is leaking. If a new bcg worked that gives some insight.

Stand the two bcgs up on the bolt face with the bolt extended(bcm and the one that worked). Do they both support the weight of the carrier? Does one feel tighter than the other?
 
#31
WeR0206 said:
The fit between the bolt and carrier could also be too tight and causing bolt drag/random short stroking that would get more frequent as it gets more fouled.
Click to expand...
Had something similar with a left handed bolt combo for a buddy.
Turns out the carrier was out of spec and removing one gas ring made it lock back properly.
Took me awhile to figure that one out and now I need to buy pin gauges.
 
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#32
If you are going replace a bolt, replace the whole BCG. not all BCGs are created equal. There are several internal sections on the carrier if out of spec will bleed gas and make this very thing happen. Just because it is a BCM doesn't mean it is GTG. They can and do put out junk just like everyone else does. And stay away from anything nitride coated. That just means that the company would make it cheaper if they could. That is the whole thing with nitride.
 
#34
You really don’t need a new bcg. Why not just buy some gas rings for $10?
Just replace the gas rings, make sure the gas key is not loose. Retorque the gas key.
If it still doesn’t work after that I’d remove the gas key and take a closer look for a blockage
 
#35
The Knuckle said:
You really don’t need a new bcg. Why not just buy some gas rings for $10?
Just replace the gas rings, make sure the gas key is not loose. Retorque the gas key.
If it still doesn’t work after that I’d remove the gas key and take a closer look for a blockage
Click to expand...
Is it actually possible to wear out gas rings in less than 2000 rounds on a brand new bcm bcg? My main training gun’s bcm bcg has over 25k on it, still passes the standing test.
 
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#37
brutus1776 said:
Is it actually possible to wear out gas rings in less than 2000 rounds on a brand new bcm bcg? My main training gun’s bcm bcg has over 25k on it, still passes the standing test.
Click to expand...
Absolutely, and it’s dependent on the gas port size. If you’re over gassed you can still run on worn out rings.
Some part of the gas system is not sealing well. Most likely rings , since a new bcg solved your problem.
 
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#40
Always a good idea to note the port size when installing a new barrel so you know what your working with but in this case he narrowed it down to the BCG since dropping in a new one fixed the problem.

*You could take your test a step further by just swapping the bolt and then just swapping the carrier to isolate if it’s rings or a gas key.
I doubt the cam pin timing is off since it worked before
 
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#41
The Knuckle said:
Always a good idea to note the port size when installing a new barrel so you know what your working with but in this case he narrowed it down to the BCG since dropping in a new one fixed the problem.
Click to expand...
Switching out to a new carrier/bcg is now one of the first things I do if im getting short stroking…it can def help to rule out a bunch of things. Ive also make the mistake of not measuring the gas port and not knowing the input and having to randomly try and rule things out. Expensive on ammo and not fun.
 
#43
The Knuckle said:
Absolutely, and it’s dependent on the gas port size. If you’re over gassed you can still run on worn out rings.
Some part of the gas system is not sealing well. Most likely rings , since a new bcg solved your problem.
Click to expand...
Thanks
The bcg I put in is also a bcm albeit with about 25k rounds on it.

What the port size need to be to not wear out rings fast in those round counts? I can’t be replacing gas rings after every class or every few training sessions if this is the problem.
 
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#44
Per gas port size… no i didn’t take apart the gas block. I assumed with swapping the bcg it wasn’t necessary. However, I will do it. Bcm and Giesselle both spec a .076 for mid length gas. I guess I can’t rule out the G barrel being out of spec until I actually check it. Can I get a decent guess on size with just a mic or do I need to order pin gauges? Any cheater tricks without pin gauges?

Still have my head scratching how I can take off the brake and all is well.
 
#45
brutus1776 said:
Per gas port size… no i didn’t take apart the gas block. I assumed with swapping the bcg it wasn’t necessary. However, I will do it. Bcm and Giesselle both spec a .076 for mid length gas. I guess I can’t rule out the G barrel being out of spec until I actually check it. Can I get a decent guess on size with just a mic or do I need to order pin gauges? Any cheater tricks without pin gauges?

Still have my head scratching how I can take off the brake and all is well.
Click to expand...
You could probably get within a few thou using a mic but I don’t know of a way to figure out exact size without a pin gauge set. Here is the one I got recently that was only $63 for 0.061-0.250”


My guess on why the brake is causing these issues, you were close to the bleeding edge without the break and adding it slightly reduced the pressure going back which caused the random short stroking. Fixed by swapping out the carrier which likely had less bolt drag internally and allowed a more full cycle.
 
#49
sounds to me like the gas port is drilled from the manufacturer for a non-adjustable gas block - too small. I suggest you open up your gas port a little (.015" -.020") so that you can use your adjustable gas block to its fullest across your possible combinations of buffer weight, buffer spring tension, and cartridge data.
 
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