My Stag Arms AR is short stroking. I've owned it for 3 years and put about 2-3000 rounds through it.  Swapping in another BCG (also from a STAG AR) seems to have solved the issue.  Is the problem really solved, or is this just a band aid fix?

What kind of ammo?

Possible the gas block got loose and now not enough gas going through?

Fouled up gas tube?

If the new BCG is lower mass, it can see it "solving" the issue for now.

Sounds under-gassed.

1) Check the staking on the gas key where it attaches to the bolt carrier. Make sure the bolts are tight and restake them properly if necessary.

2) Check the gas rings.

Poor staking if the gas key or out of spec gas rings are not uncommon in hobby guns.

If a new BCG solves the problem, then the BCG is the likely culprit. I would start there.

Ammo is Wolf Gold 223.  The BCG is the same mass as far as I can tell, It came from the same model rifle

 

slow down....you ran 2-3k with old bcg and had short stroking issues.  You replaced it with a bcg and now it seems to be ok.  is this correct?

 

how many rounds with new bcg? How many rounds inbetween with old bcg did this occur?

Under-gassing could be caused by a number of things.  Many AR owners never clean the gas tube.  Very long pipe cleaners are made that work well for this.  Did you check the gas key as previously mentioned, is it fouled or loose?  Other potential culprits are the gas rings in the bolt, they are like piston rings.  Do you have a lot of fouling in the bolt?

issue is not gas system, it will be in the carrier or bolt (extractor most likely and will be lock up time so I'd pull it apart and replace).  If OP has gone several hundred rounds with no issue then we can safely assume it's the bcg area.  Is the carrier in spec? 

did you try just swapping the bolts or the whole carrier

gas rings ok?  agree with JC, did you check the key?

OP needs to answer my previous questions first.

I read in the OP that he had a short-stroking issue, broken gas rings could cause a loss of gas pressure causing short-stroking.  A new BCG (with new rings on the bolt) would resolve this. Rings are a definite breakage/wear item, I always stagger the spacing on the rings to avoid any blow-by.  I would replace them if they appear worn or if the bolt slides too easily in the carrier.

yeah but he said he did 2-3k and had short stroking issues.  He then put in new bcg and no issues.  That should eliminate the gas system.  Totally agree the carrier key could have been loose.  I would venture to say his bolt is locking up incorrectly.  I'd like to see if he changes the just the bolt upon inspection of the carrier.  I don't know about stag but he could have a wrong o ring, 2 of them for that matter.   Hell, even his ammo could be an issue.  don't know, need more info here

 

oh and Shenandoah valley is one of the prettiest parts of America...God Country if you will

My first guess would be rings. Second would be gas system has a leak.

I have long pipe cleaners but if your gas system works well a lot of people believe there is not a need to push a long pipe cleaner through the system.

As stated, check you stakes. Good luck, figure out what caused the problem with your BCG and have it as a spare,

3 years old? Your rifle is out of date. Sell it and buy a new one.

Gas key, or gas rings are this issue 

 

3 hours ago, JC_68Westy said:

Under-gassing could be caused by a number of things.  Many AR owners never clean the gas tube.  Very long pipe cleaners are made that work well for this.  Did you check the gas key as previously mentioned, is it fouled or loose?  Other potential culprits are the gas rings in the bolt, they are like piston rings.  Do you have a lot of fouling in the bolt?

If 20,000 PSI of viciously hot gas isn't going to clean it out, giant pipe cleaners aren't going to help. I've never seen anything but problems caused by the giant pipe cleaners. Avoid them. At best they just make you feel like you did  something while perfomring a no-op that cost money. 

I have to agree, I have never once clean the gas tube. They are self-cleaning to some degree but also bear in mind that they are cheap and disposable. Everyone should have spare gas tubes gas rings and cam pins on hand

Never, ever have seen a dirty gas tube. 

 

4 hours ago, JC_68Westy said:

Under-gassing could be caused by a number of things.  Many AR owners never clean the gas tube.  Very long pipe cleaners are made that work well for this.  Did you check the gas key as previously mentioned, is it fouled or loose?  Other potential culprits are the gas rings in the bolt, they are like piston rings.  Do you have a lot of fouling in the bolt?

 Don't stick anything in the gas tube, They are basically self cleaning.

6 hours ago, myhatinthering said:

slow down....you ran 2-3k with old bcg and had short stroking issues.  You replaced it with a bcg and now it seems to be ok.  is this correct?

The old BCG initially ran fine.  It didn't start short stroking until last week.  With the new BCG the short stroking seems to have stopped.

6 hours ago, myhatinthering said:

how many rounds with new bcg? How many rounds in between with old bcg did this occur?

I only had time to run about 15 rounds with the new BCG.  The old BCG seemed to short stroke about every three rounds

Clean  the BCG and check the gas rings, I would give the chamber a scrubbing too.

Gas tubes usually fail by blowing out at a weak point or coming loose from the gas block. They should never be so dirty that they cause issues. As stated, they are self cleaning. They do however experience wear - at the gas block end especially. As such, the replacement of the gas tube should be part of your preventative maintenance program.

There is no need to clean them. Throw out the old and by the new.

Note: I do not think the gas tube is the culprit in this case based on the information at hand.

1 hour ago, tagonagy said:

The old BCG initially ran fine.  It didn't start short stroking until last week.  With the new BCG the short stroking seems to have stopped.

I only had time to run about 15 rounds with the new BCG.  The old BCG seemed to short stroke about every three rounds

How do you clean your bolt carrier group?

I very rarely clean my BCG. If I do, it’s generally a quick wipe down with a rag and then luned, reassembled, and installed. I have never had a malfunction like the OP described.

However, I have seen this malfunction type in BCGs that are not built to the Technical Data Package (TCP) specs in materials or manufacturing. It has been almost exclusively found to be a loose carrier key with sub-par staking - if there is any staking at all, followed by a very close distant second to crappy gas rings. Occasionally the issue will be tracked to cracked cam pins or other damaged BCG parts.

If you have a quality rifle that is built to meet or exceed the TDP, you will find that proper lubrication and preventative maintenance at appropriate intervals is more important to reliability of the gun than meticulous cleaning.

For those that have never stripped the BCG for a good cleaning every once in a while would probably be very surprised at how much carbon and crap accumulates in the bolt.  I have seen more than enough stoppages caused by fouled BCG's.  Granted most were not AR15, they were M16/M4.  I keep my BCG clean.  It only takes a few minutes.  I do not consider a complete BCG a replacement unit, only the components and keep spare parts.

On a whim, I bought a spare BCG on a half off sale. I do have all the spare parts for my BCG but felt that if I travel 1 hour to my range I don't want to turn around and go home when I can put in the spare BCG and then fix the disabled one at home. It is not necessary to have a spare, just a luxury. Who knows, one day I may make a build out of it.

I have several ‘kits’ I make up for each of my AR’s, with a spare bolt, firing pin, gas rings, firing pin retainers, and a few other bits. They fit into an Altoids tin, and I keep it with the rifle. 

 

I use a fishing tackle box with dividers. One is all AR parts, while the others have less parts for various firearms.

11 hours ago, High Exposure said:

I very rarely clean my BCG. If I do, it’s generally a quick wipe down with a rag and then luned, reassembled, and installed. I have never had a malfunction like the OP described.

However, I have seen this malfunction type in BCGs that are not built to the Technical Data Package (TCP) specs in materials or manufacturing. It has been almost exclusively found to be a loose carrier key with sub-par staking - if there is any staking at all, followed by a very close distant second to crappy gas rings. Occasionally the issue will be tracked to cracked cam pins or other damaged BCG parts.

If you have a quality rifle that is built to meet or exceed the TDP, you will find that proper lubrication and preventative maintenance at appropriate intervals is more important to reliability of the gun than meticulous cleaning.

If I have a rifle that is going to sit around for awhile (and lately, I haven’t shot them, so....), I do a ‘deep clean’ and lube everything for storage. Other than that, I rarely do anything other than to run a swab down the barrel. 

I don't know how many times I have posted this on gun forums but here it is again. I have heard many people say they had a failure and cleaned their firearm and were surprised how dirty it was, then worked fine after cleaning. I have still yet to hear the cause of a FTF was because the firearm was cleaned properly. If it gets shot, I clean it.

13 hours ago, tagonagy said:

The old BCG initially ran fine.  It didn't start short stroking until last week.  With the new BCG the short stroking seems to have stopped.

I only had time to run about 15 rounds with the new BCG.  The old BCG seemed to short stroke about every three rounds

bolt issue

look at alignment or replace the rings although that is awfully few rounds to have to do so.  Also, look at the extractor for any chips or fouling inside that may affect it's lock up. 

do me a favor, just swap the bolt in the carriers and see if the short stroking stopped.  I doubt its a key to be honest

I (used to) shoot my AR’s quite heavily, and have never had a malfunction of any kind. Even after heavy use. 

AR doesn't need to be cleaned as often as the mil standard applies.  I too was raised to clean and lube generously upon use however, its not as critical as folklore suggests.  Defintely clean it before 3k though.  OP, was the rifle cleaned so we can also eliminate this as a issue?