Rifle Build Help

[cody6510]

First time post, I've been reading for a while now, but have yet to answer my question. I built a new rifle, but cannot get it to run for the life of me.

 

The systems I've assembled contains the following:

18" Criterion Barrel with a rifle length gas system

Seekins Adjustable Block

JP Low Mass Carrier

BCM Bolt with JP enhanced rings installed

JP Low mass buffer and spring

AR Gold Trigger

 

The problem I'm having is the carrier is not locking to the rear on an empty mag and will not pickup the next round in the magazine. I have tried various types of factory .223 ammo. When testing I adjusted the block as Seekins advised me to, 1/4 turn at a time. I ended up with the block wide open, 4 turns out, and was still experiencing issues. For what it's worth, the gun was not even fully extracting the case until I had a little over 2 turns in the block.

 

I've double checked gas block and gas tube alignment, both of which appear to be in good order.

 

I spoke to Criterion and they were great to deal with. Their suggestion was replace all the low mass components and Seekins block with mil spec components. They believe (as I recall) my issues are related to the gas pressure still being too high when the bolt is trying to unlock, therefore the extractor is forcefully ripping the case out of the chamber and slowing the bolt down. This very well could be the problem, but my question is how does everyone else get their guns to run with low mass components? If Criterion is correct, how do I correct the timing? The rifle is for 3 gun and I would prefer to stick with low mass. I've already came to grips with the idea that this build is going to get more expensive, I just want my gun to run.

 

What suggestions/ideas does everyone have?

 

Thanks for your help.

Cody

 

 

 

 

[Steve RA]

What length is your buffer tube, carbine or rifle ???

 

[cody6510]

It's a rifle length buffer.

Edited October 14, 2018 by cody6510

[Steve RA]

Ok, removes one possibility.

[HRider]

You mentioned that you checked gas tube and block alignment, are you sure the port in the gas block is centered over the barrel gas port (front to back)? The mil-spec dimensions allow for the hand guard retainer at the gas block, most gas blocks are also machined so they will work with old style hand guards so you must leave a space between the barrel shoulder and gas block for proper alignment. 

Hurley

[Matt1911]

I ditched adjustable gas blocks long ago. They work fine in practice, but during a match they tend to close up giving a single shot rifle. 

Try a standard gas block and like was said above, make sure the holes line up properly not just side to side but also front to back. 

[TonytheTiger]

Try standard gas rings. I've had bad luck with the JP rings.

[cody6510]

6 hours ago, TonytheTiger said:

Try standard gas rings. I've had bad luck with the JP rings.

 

I can try mil spec rings out tomorrow.  I will also take a second look at the block. Right now it is against the shoulder on the. barrel. I was assuming alignment was corrct based on the dimple. 

[biggulp]

First guess would be gas block alignment.  As HRider mentioned, on most barrels, the gas port is drilled to account for a handguard retainer.  If your gas block is sitting against the shoulder, it's about ~.030 to far back.  If you don't have calipers to measure, a gas block dimpling tool, or gas block alignment pins, then pull a credit card out of your wallet and use that as a spacer.. It'll get you close enough for Government work.

[Will_M]

With low mass parts and the gas block running wide open, I'm guessing your gas block isn't aligned. I've got an almost identical setup and my gas block is only open about 50% and that's a few "clicks" past the point where it locks back on an empty mag so it'll still cycle when it gets dirty.

[TalkingMonkey]

Could try reinstalling your gas block using this tool to ensure alignment:  UNIQUETEK - AR-15 GAS BLOCK ALIGNER

[lefty o]

gas block should be .025-.030" off the shoulder of the barrel. low mass carrier, isnt your problem.

[cody6510]

So I checked my block again, and was able to tell things were in allignment by rooming the adjustment screw and looking inside the block. This was with the block against the shoulder. 

 

I figured I would try spacing the block off the shoulder and give it a try anyway.  I spaced the block about .025 off the shoulder and shot the gun today.  

 

The problem still exists and the gun is functioning exactly how it was when the block was against the shoulder. 

 

Any other thoughts? 

[Amra86]

I had a bad gas tube one time. It leaked a lot of gas out the front of the tube. What size is the gas port? I like a .101 gas port.

[Woodsk]

with a rifle length gas, I would go with a standard gas block. 

[HRider]

As stated above, gas tube could be the problem, I would try a different one. I have also seen gas leakage between the gas key and carrier cause short stroking. If you have another bolt and or bolt/carrier assembly (or a friend with some parts), start changing only one thing at the time until you find the problem. It sounds as though you are not getting enough gas to the bolt to properly operate the mechanism. If you have a mil-spec buffer spring, test it in place of the JP spring also. I have never known of an AR10 spring being packaged as an AR15 spring, but I suppose anything is possible.

I have two rifles set up similar to yours and they are very reliable. I also have an AR10 with a Criterion barrel and a Seekins adjustable gas block that works. I believe that you will find a part that is out of spec on your build. 

Good luck and let us know what you find out.

Hurley

[chase214]

As stated my buddy had the same issue and it turned out to be the gas block was installed incorrectly 

[stick]

I would leave the gas block wide open until you get the gun functioning properly.  Once you get the issue resolved, then you adjust the gas block to your specific load.  Definitely sounds like a gas issue to me.  Did the tube come attached to the gas block or did you do it?  

[MickB]

Don’t eyeball your gas block installation.

Mic the distance from the barrel shoulder to the start of the gas port. Now mic the distance from the back of the gas block to the start of the gas hole. You will more than likely have a 0.02-0.03” difference.

Use a feeler guague up against the barrel shoulder to ensure correct offset.

Another trick is to cut the end of a toothpick and drop it into the gas block hole (make sure your gas metering screw is all the way out). Position the barrel with the gas port facing down then slip the gas block on upside down. Turn the assembly right side up and look down the bore. Wiggle the gas block around. When the holes are aligned the toothpick will drop from the gas block into the barrel. Tighten gas block. The toothpick piece should fall out of the barrel, if not use a rod to push it out. All done and gas block will be perfectly aligned.

 

Mick

[Steve8140]

Any chance the buffer spring is too stiff for the low mass BCG? Maybe not allowing it to travel far enough back.