Gas port size matters??

[Hopar]

My build is AR-15
Barrel 20" Krieger with matching JP bolt
Gas port size .0865
GP to muzzle 6.5"
GP to chamber 13.5"
Aero precision upper & lower
Buffer weight 3oz
Basic buffer spring
BC and charging handle are from previous built
Superlative Arms adjustable gas block

It shoots but does not lock back on the last round or load the next round.

Found GP sizes online that mine seems to be smaller.

Based on your experience, is .0865 small for what I have and is the cause of the issue?

TIA

 

[357Max]

My build is AR-15
Barrel 20" Krieger with matching JP bolt
Gas port size .0865
GP to muzzle 6.5"
GP to chamber 13.5"
Aero precision upper & lower
Buffer weight 3oz
Basic buffer spring
BC and charging handle are from previous built
Superlative Arms adjustable gas block

It shoots but does not lock back on the last round or load the next round.

Found GP sizes online that mine seems to be smaller.

Based on your experience, is .0865 small for what I have and is the cause of the issue?

TIA

No that port is GTG.
My 20" Proof RL gas is also .0865

Assuming the basic build checks are correct such as gas block alignment over port, then try the following.

Have you adjusted the gas block open for more gas? Ad 3 clicks open & fire 1 round, rinse repeat till it locks back, then add 2-3 more clicks.

 

[TonyTheTiger]

.086 is reasonable for a 20" RLGS barrel. .093 is pretty standard for 18" RLGS.
I would look real hard at GB alignment before I touched anything else.

 

[Hopar]

No that port is GTG.
My 20" Proof RL gas is also .0865

Assuming the basic build checks are correct such as gas block alignment over port, then try the following.

Have you adjusted the gas block open for more gas? Ad 3 clicks open & fire 1 round, rinse repeat till it locks back, then add 2-3 more clicks.

.086 is reasonable for a 20" RLGS barrel. .093 is pretty standard for 18" RLGS.
I would look real hard at GB alignment before I touched anything else.

I removed and reinstalled gas block twice. Very carefully making sure it is lined up and directly over the port.
I have sprayed carb cleaner through the GT from the chamber side and verified with a bore scope that liquid comes out freely.

I have adjusted the GB twice and shot every 4 clicks, all the way open and to less open.

I have done the bolt catch test and fail that too.

FWIW, The one round that fires and ejects, it lands between 3 to 4.

 

[Hopar]

I am going to try it on Sunday again and see if I can find anything.

 

[hlee]

Can you detail this ‘bolt catch test?’

 

[msgriff]

Just to be clear, full open on a SA AGB is 4.5 rotations or 18 clicks CCW from closed. Beyond that, you're in bleed-off which delivers less gas to the BCG.
Can you manually engage the bolt catch?

 

[supercorndogs]

Doest pick another round either so most likely short stroking.

 

[mtrmn]

Just to be clear, full open on a SA AGB is 4.5 rotations or 18 clicks CCW from closed. Beyond that, you're in bleed-off which delivers less gas to the BCG.
Can you manually engage the bolt catch?

THIS^^
Insert an empty mag and manually pull the charging handle as far back as it will go. Does it lock back? If not, look into the ejection port and see if the face of the bolt will go past the bolt catch. In the most rearward position, the bolt face should be at least 1/8" behind the bolt catch.

 

[Hopar]

THIS^^
Insert an empty mag and manually pull the charging handle as far back as it will go. Does it lock back? If not, look into the ejection port and see if the face of the bolt will go past the bolt catch. In the most rearward position, the bolt face should be at least 1/8" behind the bolt catch.

Yes it locks back and goes back at least 1/8".

 

[Hopar]

Can you detail this ‘bolt catch test?’

 

[Hopar]

Just to be clear, full open on a SA AGB is 4.5 rotations or 18 clicks CCW from closed. Beyond that, you're in bleed-off which delivers less gas to the BCG.
Can you manually engage the bolt catch?

I have done this.

At a certain point around 20 clicks it ejects and the case lands around 3 to o'clock.

If I go CW, it fires but doesn't eject at all.

 

[mtrmn]

The majority of 223/556 20" barrels I've seen listed have port diameters in the low .090's like .092 -.093. Since you have an adjustable gas block it wouldn't hurt to enlarge yours somewhat since you can always adjust it down. JMHO

Also look into the magwell with the bolt locked open. See if the gas tube terminates halfway of the cutout for the cam pin like in this picture.
This usually applies more to large frame guns since the small frame parts are actually standardized but you never know until you check it for yourself. Check the screws for the gas key for looseness and proper staking.

 

[357Max]

Barrel 20" Krieger with matching JP bolt

I didn't notice this the first time (JP bolt). Their bolts are great, but the 1 piece gas rings suck ass.

First thing I do to JP bolts is replace the rings with a 3 piece Sprinco set.
Also since the carrier is out of an old build, check to make sure the gas key screws are tight. The key screws can and will brake, yet look fine because the head is staked in place. It's also possible the JP bolt tail fits looser in the tail bore of the carrier (more gas leakage). Is the old carrier a coated one like a Fail Zero?

Order a set of these. Meanwhile you can rob a worn out set of rings from an old bolt to try.

Sprinco AR15/M16 bolt gas rings-Sprinco AR15 bolt gas rings

 

[Hopar]

The majority of 223/556 20" barrels I've seen listed have port diameters in the low .090's like .092 -.093. Since you have an adjustable gas block it wouldn't ,..............

Mine looks exactly like your post.

 

[Hopar]

Reinstalled the gas block, cleaned in&out, made a spring measuring jig and even with the shortest and lightest preassure spring, STILL, doesn't lock back or load the next round.

Next, I will take my buddy's rifle and try my upper and buffer system with his.

 

[Top O' Texas]

What brand of gb is it? I've had new 1's that did not work! If the gb is gtg, then it's dragging somewhere? I've never had any issue with 556 barrel gas ports regardless of size. The chamber can be tight and need more rounds? That I've experienced with 65gren and 1 6.8spc barrel.

 

[Emerson0311]

Factory ammo?

 

[Naaman]

Factory ammo?

I'm wondering about ammo as well....

 

[tucaz]

Doest pick another round either so most likely short stroking.

or out running the mag

he said it has a 3 oz buffer, that might be too light.

 

[supercorndogs]

or out running the mag

he said it has a 3 oz buffer, that might be too light.

Hopefully in the last three months it's been resolved. Reading the thread I would still think short stroking is more likely than out running the mag.

 

[Torchy]

Let us know if you fixed it op.

 

[MSTN]

Gas ring...

 

[Hopar]

Factory ammo?

Yes. Hornady Black 75 grain

 

[Hopar]

Sorry been busy with work and only two times at the range with this AR. The issue "was" getting better.
I ordered two springs (20% lighter and 20% heavier)(yellow and white) from Sprinco. I did the gas ring test and it passed.

Installed the light springs, it was working OK. Ejects to 3-5, sometimes it would lock back after the last round. For the last 50 rounds, it doesn't lock back after the last round and about every 2-3 shots I need to cycle the bolt manually.

Next time I will take my buddies AR and just changed my upper and lower with his. I will also try 556 ammo and see if makes a difference.

 

[Top O' Texas]

Sorry been busy with work and only two times at the range with this AR. The issue "was" getting better.
I ordered two springs (20% lighter and 20% heavier)(yellow and white) from Sprinco. I did the gas ring test and it passed.

Installed that springs, it was working OK. Ejects to 3-5, sometimes it would lock back after the last round. I want to say the last 50 rounds doesn't lock back after the last round and about every 2-3 shots I need to cycle the bolt manually.

Next time I will take my buddies AR and just changed my upper and lower with his. I will also try 556 ammo and see if makes a difference.

Forget about the locking back on last round for now. Does it function properly (eject and feed) the whole mag? If so there could be 1 of 2 issues left? 1 is there's not quite enough gas for last round hold open? 2 is the catch is working correctly with the mag? Even though it will manually hold open when charging on an empty mag, but the catch itself isn't made well enough?

 

[Cfshooter]

I vote for gas port too small.

 

[Hopar]

Forget about the locking back on last round for now. Does it function properly (eject and feed) the whole mag? If so there could be 1 of 2 issues left? 1 is there's not quite enough gas for last round hold open? 2 is the catch is working correctly with the mag? Even though it will manually hold open when charging on an empty mag, but the catch itself isn't made well enough?

No it does not.

 

[hkguns]

Colt standard for 20" rifle length gas is .093 +/-.

I'd get rid of the AGB and install a regular one to see what happens, I'd also call the barrel MFG and ask about the gas port size. Krieger does very good barrels, but anyone can make mistakes.

 

[Top O' Texas]

No it does not.

Break open the upper, with the chargehandle in place press with your fingers to unlock the bcg, then at the back of the bcg put it back into battery. Trying to check tube to bcg function as not to drag. Also you can try and see if the tube is clear by using compressed air (cans) by using the nozzle extension (supplied with the cans) inserting it into the tube via bcg space, block off the chamber and listen/feel air coming from muzzle. Not so much an alignment issue, but to check gb. No obstructions in the gas port or the block you know of?

 

[Hopar]

Break open the upper, with the chargehandle in place press with your fingers to unlock the bcg, then at the back of the bcg put it back into battery. Trying to check tube to bcg function as not to drag. Also you can try and see if the tube is clear by using compressed air (cans) by using the nozzle extension (supplied with the cans) inserting it into the tube via bcg space, block off the chamber and listen/feel air coming from muzzle. Not so much an alignment issue, but to check gb. No obstructions in the gas port or the block you know of?

I will try this again during the weekend.

During my troubleshooting, I have confirmed that air goes through the GT. In addition, I have cleaned the inside of the GT with carburetor cleaner to clean any built up carbon.

When I push on the BCG, it goes it smooth and locks. I don't hear any grinding or rubbing of two surfaces. When I pull it, it gently releases and slides out.

 

[Top O' Texas]

I will try this again during the weekend.

During my troubleshooting, I have confirmed that air goes through the GT. In addition, I have cleaned the inside of the GT with carburetor cleaner to clean any built up carbon.

When I push on the BCG, it goes it smooth and locks. I don't hear any grinding or rubbing of two surfaces. When I pull it, it gently releases and slides out.

It's down to port size being the issue or the gb itself.

 

[Hopar]

It's down to port size being the issue or the gb itself.

I will try my buddies lower and upper on mine, before I change the GB.

 

[Cfshooter]

It's down to port size being the issue or the gb itself.

I’m all in on port size

 

[tucaz]

carbine buffer set up with a 20" was never all that reliable anyway. its been a long time but i didnt stay with it. if you really need a carbine butt stock, try a vltor a5...once you confirm gas port sizing.

 

[Hopar]

Most likely the issue is solved.

I removed the Adj. GB and installed the one from Krieger. Before taking off the AGB, I noticed the gas tube is going into the receiver in a subtle angle and when I installed the regular GB the gas tube goes into the receiver parallel to the bore.

I shot 85 rounds this weekend (20 factory and 65 my first reloads) and maybe first 5 rounds were some issues, but the rest worked like a charm and the BC held back every time and my chamber, BC and the bolt was pretty clean after the session.

So here is my theory, The GT going in an angle did not sit 90 degrees to the back of the gas key. Hence, some of the gas escaped into the receiver and the bolt carrier did not move all the way back due to lack of pressure. This also explains why my chamber, bolt, BC was always filthy.

I will update if the issue comes back.