AR Tuning question

[brutus1776]

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!

 

[slodsm]

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.

 

[TonyAngel]

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!

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.

 

[WeR0206]

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!

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.

 

[brutus1776]

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.

 

[mettee]

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?

 

[brutus1776]

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

 

[brutus1776]

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.

I haven’t weighed the buffer it’s self but it’s whatever comes in a stock bcm lower.

 

[mettee]

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.

 

[USK308]

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.

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.

 

[brutus1776]

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.

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.

 

[The Knuckle]

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.

 

[mettee]

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.

 

[brutus1776]

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 ?

 

[The Knuckle]

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.

 

[msgriff]

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.

 

[BAMCIS]

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

 

[brutus1776]

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.

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.

 

[WeR0206]

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.

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.

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

 

[tomme boy]

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.

 

[mettee]

Tip on the gas tube could also be worn if you didnt replace that. Usually not an issue unless its bad.

 

[fedupflyer]

Tip on the gas tube could also be worn if you didnt replace that. Usually not an issue unless its bad.

Yeah, If he truly shot out a BCM barrel the gas tube should have been replaced also.

 

[mtrmn]

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