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Discussion Starter #1
I just replace my gas block with an adjustable gas block. It's for my 16" 6.8 spc it has an arp barrel. The issue I'm having is the bolt won't lock back. I was using my reloads (which were made before the gas block switch). I also tried factory sst 120gr. I also switched lowers and buffer tubes same thing. It worked great before the gas block switch. I noticed some carbon in front of the glad block (towards the muzzle). Did I get a bad gas block or is there something else. Any help would be great.


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You have it open all the way and it won't lock back? What kind of gas block is it?
 

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Most builders will put a little gasket sealant on the gas block. Did you get good alignment on the gas hole?
 

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Why would you switch it? I'm of the opinion a lot of these heavy buffers and adjustable blocks are solutions looking for problems that end up causing problems. I use basic mil spec setups on all four of my AR's and have never had any issues with any of dozens of ammo combos I've tried not locking back or cycling.

This is a case of don't fix what isn't broke.

You have carbon on the muzzle because that's where the bullet exits.
 

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Discussion Starter #7
I made sure when I was tightening I move it back and forward so I can feel the indent on the barrel.


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Discussion Starter #8
I switched it because I have a suppressor locked up for my sbr and plan on using it on the 16" also.


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I never trust the dimple on the barrel, call me a bit OCD. I measure the distance on both the barrel and block then select a proper size feeler gauge based on the measurements. That takes care of distance but for clocking the block I use a piece of masking tape and mark the center line on the barrel for clocking.

As said before I would double check the block alignment and check for any burrs in the tube.

In addition when ever tuning with a adj block you start with standard weight buffer and standard strength spring. Then open block a few clock off closed then open click by click till it operates probably.

Good luck
 

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Discussion Starter #10
I will take a look at the gas block again. I took it off after I started having issues. I will look over it again really good. When I start adjusting I always start from close and turn a few clicks.


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IF anything would have cycled it to lock back that would have been the 120SST it's far from under powered.
I have the same gas block on my 20" ARP Barrel and have noticed a slight amount of leakage around the detent but has never interfered with function.
Just a few things to look at.
1) When you installed the gas tube to the block did you look through the set screw hole in the block and verify that the port in the tube was centered in block port and not partially obstructing it causing a restriction?
2) Is it the original gas tube used from the prior set up, if not you may want to verify the I.D. is of the correct size. I have never encountered the issue but have seen it mentioned by one of the mods on this forum.
3) With the bolt removed from the carrier slide it by hand into the forward position to feel for binding or resistance with the gas key. If resistance is found you will have to determine if it is from being canted or bent or is meeting resistance due to tolerance issues with the gas key.
 

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I will take a look at the gas block again. I took it off after I started having issues. I will look over it again really good. When I start adjusting I always start from close and turn a few clicks.


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Sounds like you did everything right.

Are you still feeding and ejecting fine, just not last round bolt lock back?
 

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Discussion Starter #13
Every round the bolt will not lock. The mag can have between two rounds or full with 10. I bought the gas block at midway so Im thinking about returning it just to make sure it's not it. I bought a gen I I for the sbr and it doesn't have any issues. I didn't have to use any sealant on it. Also the original gas block did have any sealant either


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So how may clicks from the closed position is the most you have tried. Mine is running 7 clicks from fully closed on S&B 110 fmj. That is using the JP low mass carrier. I would think a standard carrier would require more gas so would have to be open a good bit farther.
 

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Discussion Starter #15
I opened it all the up and still wouldn't function. Basically one click before the screw doesn't click any more.


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I opened it all the up and still wouldn't function. Basically one click before the screw doesn't click any more.


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So it cycles fine, ejects and picks up another round but won't lock back on an empty mag? Just trying to make sure we are all on the same page here.
 

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there is no need for an adjustable gas block on a 16" rifle even with a suppressor. i run a suppressor on my 16" 6.8 and 5.56 rifles with a standard gas block all the time and have no issues. all you did was create problems with a solution you did not need as you found out by posting this.

that being said i do run adjustable gas blocks on my 4 SBRs.

if you want to avoid the extra gas blowback when running suppressed get a piston system.
 

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So it cycles fine, ejects and picks up another round but won't lock back on an empty mag? Just trying to make sure we are all on the same page here.
My thoughts too, didn't quite understand the OP's previous explanation.
 

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I run the same block on my 12.5 arp suppressed all the time. Works great and has reduces trigger fouling (from excess dirty gasss) and cycle pressure.

I would check the whole thing from gas port hole, allignment, everything.

I'm no pro, but you said 2-10 rounds in the mag.... You have tried it with 1 round right? Sorry if a too ignorant question.
Cheers
 
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