[DarinCraft]

My target AR constantly short cycles.

1. Numerous times when shooting I will have an empty chamber and will have to cycle it manually. It appears the bolt is not cycling back far enough to pick up the next round in the mag.

2. Every time the last round is fired the bolt will hold open on the follower and not on the BHO. I have to manually pull the bolt back another 1/2" before the bolt will catch on the BHO.

3. It will ALWAYS eject the spent case and never stovepipes.

4. If I have to extract an unspent round for some reason, it take a TON of effort to pull the bolt from battery as if the chamber is too tight.

SPECS-
Pre built DPMS upper with a 16" SS bull barrel.

I have put two gas blocks on it, cleaned the gas tube and gas port.

CMMG lower with DPMS LPK

Carbine buffer and spring.

I have swapped this upper on my other AR with no change in performance. I am starting to think either the headspace is too tight or there is something wrong with the gas system.

Please help.
Darin

 

[Jpyle]

1. Did the problem appear gradually with infrequent short cycles increasing to the current situation or did it appear suddenly?

2. Is the gas block seated properly? Any visible signs of gas escaping under the port?

3. Do you shoot a lot of steel cased ammo? Are the rims being torn off the cases?

4. What type and weight buffer? H1, H2, etc

Difficult to diagnose over the internet but given the problem can be repeated on a different lower points to the upper. Quite possibly the gas block port is not 100% in line with the barrel port. It happens, many folks simply line the gas block up with the shoulder cut in the barrel and pin it down...sometimes a little finesse is required. Goes without saying, make sure the gas key is securely attached to the carrier and there is no "mushrooming" of the tube end.

For the sticky extraction, try a chamber brush and some Hoppes. Steel ammo, if you shoot it, can leave a residue in the chamber that can gum up when reheated and adhere the brass to the chamber walls.

 

[willfully armed]

Are the gas key screws tight? Are the o rings lubed?

 

[DarinCraft]

1. Did the problem appear gradually with infrequent short cycles increasing to the current situation or did it appear suddenly

Has been a problem since the first day of operation.

2. Is the gas block seated properly? Any visible signs of gas escaping under the port?

There are signs of gas escaping back toward the receiver, however happened with both gas blocks. There is a relief dome inside the gas block that allows a shadow of carbon to be left behind on the barrel. The gas port on the bbl is dead center of that shadow.

3. Do you shoot a lot of steel cased ammo? Are the rims being torn off the cases?

I shot steel for a couple of hundred rounds, but switched out. The rims were not torn off. I would have to use a rod to get the case out.

4. What type and weight buffer? H1, H2, etc

Standard M4 civilian buffer and spring that comes with the cheapo collapsible stocks.

Difficult to diagnose over the internet but given the problem can be repeated on a different lower points to the upper. Quite possibly the gas block port is not 100% in line with the barrel port. It happens, many folks simply line the gas block up with the shoulder cut in the barrel and pin it down...sometimes a little finesse is required. Goes without saying, make sure the gas key is securely attached to the carrier and there is no "mushrooming" of the tube end.

For the sticky extraction, try a chamber brush and some Hoppes. Steel ammo, if you shoot it, can leave a residue in the chamber that can gum up when reheated and adhere the brass to the chamber walls.

I'll try that. Thanks for the help JPYLE, I really appreciate it

 

[DarinCraft]

Are the gas key screws tight? Are the o rings lubed?

The screws are tight, but what O rings?

 

[willfully armed]

The o rings on the bolt, inside the carrier.

Clean your chamber as well. Brakleen does exceptionally well on removing chamber deposits.

 

[DarinCraft]

The o rings on the bolt, inside the carrier.

Clean your chamber as well. Brakleen does exceptionally well on removing chamber deposits.

Wow well I feel stupid because I had no idea there were o rings there. I'll check tomorrow and post up. Thanks WA

 

[Quentin]

Don't go looking for rubber O-rings though, just three steel gas rings on the bolt.

 

[Jpyle]

Don't go looking for rubber O-rings though, just three steel gas rings on the bolt.

 

[Txhillbilly]

I'd say you have a buffer spring problem. I had the same issue with my DPMS LR-260,it was jamming,short cycling,and the bolt wouldn't stay open after last shot.
I could manually pull back the bolt,and it would lock open,but firing the rifle it wouldn't.
I ended up cutting 3 coils off the buffer spring,and all issues disappeared. The rifle works like a champ now.

 

[DarinCraft]

This is the gas port on the barrel. You can see the shroud of carbon around the port and some blow by on the shelf.

Another view

gas port on the block. You can see some blow by on the gas tube.

Another view of the gas block

Receiver end of the gas tube

From what I can see it looks like I have some sealing problems in the gas block. Now is it because the ports aren't lining up, or another problem? How do I insure the two ports line up?

Thanks for the help guys

 

[Quentin]

Well if you temporarily remove the gas tube from the block then a bent paper clip through the gas block will help you line up with the barrel's gas port. I read that somewhere and think it was Sniper03 who gave that tip.

 

[Jpyle]

Darin:

What Quentin suggested will work fine. An alternative albeit more lengthy process is to first make sure the opening in the tube lines up with the port in the gas block when pinned...this will determine if the gas block and the tube are in spec or need replacing. Now remove the tube from the gas block and align the port in the tube with the port in the barrel so that the center of the tube port is centered with the barrel port. Using a black sharpie make an index mark on the tube and barrel to be used to align the gas block. Insert the tube back in the gas block, making sure the port is facing down towards the barrel, and pin it securely. Slide the gas block onto the barrel and align the index mark on the tube with the index mark on the barrel making sure the tube is aligned straight to the opening in the barrel nut and receiver. Tighten down the gas block. At this point the barrel, tube and block ports should be in perfect front-to-back and side-to-side alignment. Relying on the shoulder cut in the barrel to align the gas block can be problematic if the ports on the barrel or gas block are the tiniest bit out of spec...not uncommon on some barrels, also that shoulder is cut to allow for a hand guard cap so the gas block is actually about 1/16" forward...

A little opinion here, those 2 screw blocks are notorious for leaking, I would recommend a different attachment type if possible.

 

[DarinCraft]

Thank you for the great help. I will get a different block and follow your procedure. I am also getting an a2 stock assembly. If that didn't work then the spring is getting cut. Thank you again guys.

 

[Jpyle]

Thank you for the great help. I will get a different block and follow your procedure. I am also getting an a2 stock assembly. If that didn't work then the spring is getting cut. Thank you again guys.

If you get an A2 stock you will need a new tube spring and buffer anyway. You do have a carbine length spring and buffer now right?

 

[DarinCraft]

Yes. I put the carbine on there because that was all I had. Thanks again.

 

[DarinCraft]

I have to say thanks to JPyle. He was correct. There was too much blow by around the gas tube. It was also leaking between the barrel and the gas block. I put a YHM GB on it and the problem went away.

Thank you to JPyle and all those who replied.

 

[fsted2a]

Well if you temporarily remove the gas tube from the block then a bent paper clip through the gas block will help you line up with the barrel's gas port. I read that somewhere and think it was Sniper03 who gave that tip.

I am glad the problem has already been resolved, but I have another method of lining the gas block on the barrel. I take a straight edge and draw a line with some white out from the port to past where the gas block will end. Then I make another line from the port on the gas block to where it mounts on the barrel. When I put the gas block on the barrel, I just make sure the lines touch and I am good to go. The white out can be wiped off without scratching the barrel and no one knows it wasn't mounted at the factory.