AR10 cycling problem

[Barretn]

I am new to ARs and having an issue with an AR10 308 I put together.

Purchased a 308 upper build kit from Delta Team Tactical. 20" barrel, aero upper. I do not recall the barrel or bcg manufacture.

I put this together took it out and shot about 5 rounds very slowly and it cycled as expect then stopped cycling. Cleaned it checked the gas port was properly aligned and clean. Took it back out no change.

Thought maybe it was under gased. Ordered a JP adjusted gas block and new tube.

Followed JP's instructions on setting it up and still will not cycle. Even when fully open. Shot 40 rounds adjusting the block 1/2 turn at a time.

My gas port on my barrel is approximately 0.080 7.125 from the muzzle barrel at block is 0.75.

Could my port be to small or is there another thing I need to try? Any thoughts or ideas are greatly appreciated.

 

[Mark M.]

There are only so many things that it could be, 1st make sure that the bolt Carrier cycles freely using the charging handle, if it doesn't, there could be a misalignment in relation with the carrier and the stock tube, if this is not the issue, I would disassemble the upper, making sure all ports including the barrel and tube are clean and reassemble double checking the gas block alignment

 

[dfanonymous]

I am new to ARs and having an issue with an AR10 308 I put together.

Purchased a 308 upper build kit from Delta Team Tactical. 20" barrel, aero upper. I do not recall the barrel or bcg manufacture.

I put this together took it out and shot about 5 rounds very slowly and it cycled as expect then stopped cycling. Cleaned it checked the gas port was properly aligned and clean. Took it back out no change.

Thought maybe it was under gased. Ordered a JP adjusted gas block and new tube.

Followed JP's instructions on setting it up and still will not cycle. Even when fully open. Shot 40 rounds adjusting the block 1/2 turn at a time.

My gas port on my barrel is approximately 0.080 7.125 from the muzzle barrel at block is 0.75.

Could my port be to small or is there another thing I need to try? Any thoughts or ideas are greatly appreciated.

It could be. You can try measuring the gas port diameter with a pin gauges or the back of some wire drill bits. I would be sure about if the weight of your buffer and the spring type, and wait and shoot it more to see if it loosens up. I've seen the lock up between bolt and barrel be pretty tight before and mess up the gas operation causing short stroking. It wouldn't hurt to your bolt a couple hundred times to help break it in before asking a gun smith to open up any gas ports..

 

[mtwarych]

Make sure your gas block isn't moving towards the muzzle due to recoil.

 

[Kansaswoodguy]

AR-10 have no set standards my gun uses a 1/4" longer gas tube than an AR-15 rifle length tube and will not cycle with a AR-15 tube installed. Also I have had burs inside the gas port cause short cycling remove gas block and hand turn a drill bit into the port is all it should take to clean it out.

 

[Hawkeye Pierce]

AR10 is not like the 15, uppers and lowers are not totally interchangeable. Go through the recommendations already said and double check all components that they are set up for AR10.

 

[Farmerboy78]

Check the bcg to make sure the gas key is firmly seated and tight... also make 100% sure the gas block hole is perfectly aligned with the barrel hole. Have seen both of these issues cause that.

 

[chasjr1]

I am new to ARs and having an issue with an AR10 308 I put together.

Purchased a 308 upper build kit from Delta Team Tactical. 20" barrel, aero upper. I do not recall the barrel or bcg manufacture.

I put this together took it out and shot about 5 rounds very slowly and it cycled as expect then stopped cycling. Cleaned it checked the gas port was properly aligned and clean. Took it back out no change.

Thought maybe it was under gased. Ordered a JP adjusted gas block and new tube.

Followed JP's instructions on setting it up and still will not cycle. Even when fully open. Shot 40 rounds adjusting the block 1/2 turn at a time.

My gas port on my barrel is approximately 0.080 7.125 from the muzzle barrel at block is 0.75.

Could my port be to small or is there another thing I need to try? Any thoughts or ideas are greatly appreciated.

You need a different gas tube {https://www.armalite.com/product/10007010-ar10-rifle-length-gas-tube/} it will fix your problem, the standard gas tubes are 1/4" too short

 

[tim_w]

So to understand. You first time out with the gun it cycled fine for 5 shots then stopped cycling using the same ammo.

You mention AR10 then the upper you used. Many people use "AR10" to describe all large frame ARs. As you can see their are basically two main variants. Those patterned after DPMS/Knight and those after Armalite's AR10. So what is your lower give us and manf or do you know which pattern? Overwhelmingly DPMS is the most used They Aero upper you have is DPMS patterned. I am hoping you just used AR10 as a descriptor. Assuming that is not an issue moving on..........

You have a 20" and by your measurement its a rifle gas port. Normally the port for 308win is around .085- .098 for .750 diameter rifle length 20". If you have or can get a drill bit set that is on wire size try bits between size 40 (.098) thru 46 (.081) (large number is smaller diameter). Drill bits are the fast cheap way to get very close to spot on. Use the shaft section. But the size you estimated is inside the normal range. Just remember small changes in port size create large charges in gas vol and @ carrier psi.

Given your work on the gas block etc it does not sound like that. Same with the gas tube. Not saying the tube is correct but given the fact it was running and then stopped indicates something changed. Given that lets just temp rule out those two out and check some other things that will be clear if they are the cause.

Farmboy is onto something, as other than what we temp rules out above, there is only one area that can really change and its in the bcg. The most likely one is the gas key's allen head screws have come loose or sheared..

First look and see if the screws /key were actually staked. Ideally they would be staked and counter staked. If they are not that needs to be done when finished. But either way check the screws with a tq driver. Its possible they used cheaper weaker grade 5 screws. Grade 8 is proper. 55-58 inch lbs. I use and very much prefer torx head grade 11.5 @ 60inlb screws. No risk of stripping the allen head. They have ribbed outwall which grabs staking better. Also no standard vs metric size issues.

. Actually check it against bare glass to see its flat. If it came loose and was fired it could have been warped. If it is order a new one and a couple extra is not a bad ideal to have around. I also take the gas key once its off and lightly sand its base over a piece of glass to insure its flatness for mating to the carrier flat

If you find a screw/s loose not sheared replace them regardless as they could gave stretched.

The other less likely is the gas rings on the bolt tail are loose to the carrier fit. Stand the carrier on end with the bolt extended. It should not compress under its own weight. It should take some downward force from your hand.

Another recommendation (off topic) to ensure smooth initial operation of a freshly built AR or any gas gun for that matter and what I did with every AR I built and sold was to oil the bcg up heavily and then manually cycle it by the charging handle a min of 500x. On high ends builds I did 1000 cycles. It laps and smooths all the contact areas and breaks in the action so it cycles much smoother from the start instead of taking 10-20 mag loads.

I used to have the whole family sitting watching TV with towels on the floor cycling ARs while we watching TV 3 days a week to keep caught up.

So first thing is lets make sure you have same pattern upper and lower.

Next check and see what drill bit shaft fits in the gas port.

Inspect your bcg screws and staking.

 

[Barretn]

All,

Thank you for your input. I have the rifle cycling currently. I enlarged the gas port and used CLP oil. The combination of the 2 allowed me to adjust the gas flow and get it to cycle properly.