Short stroking AR15, what am I missing?

[someguy2800]

I have an AR15 that I've been playing with all summer. I built it from an anderson lower, a BCA side charging upper, APF parts kit, and a Leapers A2 rifle stock and buffer. I started with a mid length pencil16", then a heavy 20" rifle length, then a medium weight 16", and now I just got a real light weight Ballistic Advantage 20" fluted 223 wylde barrel that I put on today.

The gun has always been spotty with locking back on the last round but now with the BA barrel it short strokes so bad that with all but the hottest loads you have to cycle the bolt by hand with each round as it doesn't cycle far enough to pick up a round. I had some real light loads that would half cycle and rechamber the spent round. I took it apart and the gas block hole is centered over the gas port in the barrel and the hole in the gas tube is open into the gas block. The gas tube is not plugged, I can blow through it freely and stick a wire down it. Gas port in the barrel is .090". I just cleaned the action bolt and carrier and oiled them up good. The buffer tube and spring are greased. The bolt carrier moves freely and smoothly it's whole length of travel. Loads are warm, 1/2 grain under max and primers show healthy pressure. Factory ammo also works the same.

Today I experimented with the buffer. It's a rifle buffer so it's 5 oz and has 5 weights inside. I removed two weights and put a piece of rubber hose in there place of the same length, not much difference. Then I replaced one more weight and the aluminum spacer with a rubber hose and cut an inch off the buffer spring. Then it would cycle but not lock back. Finally I emptied the buffer completely and put it back together. It functions perfect now with an empty 1.7 oz buffer.

Is there something I'm missing in the gas system? Something to check in the bolt carrier? It's working fine now but it would be nice to get it working with the proper buffer weight and spring and have a bit more margin of reliability. This is a varmint gun, not a defense gun, so it's not that critical but it's annoying having to fiddle with it when shooting.

 

[ontarget]

Is the gas key on the BCG tight? Also never ever heard of greasing the buffer or spring. That could be slowing your buffer down dramatically and causing your problem.

 

[someguy2800]

The grease on the spring was a tip I read somewhere for stopping the spring from twanging in the stock. Works really well for that.

 

[LoonWulf]

I use grease on my buffer spring for the same reason.

If your using the same bolt and gas tube id check the fit between them, id also check for a leak out of the key, or worn gas rings.
Also with the upper off the gun make sure everything moves smoothly with little effort.

Oh almost forgot, check that your gas tube lines up properly with your gas block as well. I had one that pinned 1/16th of an in or so off.

 

[ontarget]

Grease may dampen the twang of the spring but IMHO it could also slow the buffer thereby causing a short stroke condition. If it works then by all means do it, I've just never heard of that. I'm also not the super AR guru either. Learn something new every day.

 

[FL-NC]

Make sure the gas key is tight and staked. Also do a drop test on the BCG, and make sure ring gaps are staggered.

 

[farmerboy78]

What is the gas length of the barrel? Sounds like something is either not lined up correctly, gas port is too small, or tube is gunked up

 

[someguy2800]

I use grease on my buffer spring for the same reason.

If your using the same bolt and gas tube id check the fit between them, id also check for a leak out of the key, or worn gas rings.
Also with the upper off the gun make sure everything moves smoothly with little effort.

Oh almost forgot, check that your gas tube lines up properly with your gas block as well. I had one that pinned 1/16th of an in or so off.

The gas tube and gas block line up well. I don't have any frame of reference of how tight the fitment between the gas key and the tube should be. When you slide the two together there is a tiny bit of movement between the two. Probably a couple thousandths clearance Everything move very smoothly, no hangups at all operating it by hand.

Make sure the gas key is tight and staked. Also do a drop test on the BCG, and make sure ring gaps are staggered.

Never heard of the drop test, had to google it. With the carrier group standing on end it takes about 2 lbs of force on the carrier to cam the bolt. I checked and the gas key is tight. Took the bolt out and the rings were staggered.

What is the gas length of the barrel? Sounds like something is either not lined up correctly, gas port is too small, or tube is gunked up

Its a rifle length gas system on a 20" barrel.

One small thing I noticed when I was taking it back apart to check tonight, there is a small amount of gas leakage between the gas tube and the gas block. There is some soot around the gas tube where it goes into the block and there is a tiny bit of soot blown onto the top of the barrel. I'm assuming this is normal since none of these parts have any positive seal on them. I never would have noticed it if not for being a stainless barrel.

 

[LoonWulf]

See if there are any scuffs or dings on the bolt carrier suggesting contact during cycling. Try dry fire then flex the upper down and pull the charging handle. While its rare i had one upper and lower that would slightly bind if you did that. Worked fine untill i converted the upper to 300aac and tried to run subsonic. Then it hung up enough to short stroke.

 

[Tirod]

Gas leakage under the block is normal. However, the port is located by assuming it will be spaced with a handguard cap about .030". Butting the gas block against the shoulder may cause overlap choking the port. The barrel port may be large enough but the block port block can be smaller, adding to the effect.

 

[Theohazard]

make sure ring gaps are staggered.

That one is a myth. There’s no reason to stagger the gas rings. I was taught that myth in the USMC, and so were many other service members, so unfortunately it persists.

Here is a good article on the AR-15’s gas system. It may help the OP diagnose his problem and it also mentions this gas ring myth.

https://primaryandsecondary.com/understanding-the-ar-15-gas-system/

 

[chicharrones]

Never heard of the drop test, had to google it. With the carrier group standing on end it takes about 2 lbs of force on the carrier to cam the bolt. I checked and the gas key is tight. Took the bolt out and the rings were staggered.

That test is done with the cam pin installed? For some reason I'd think the cam pin should be removed to test the bolt rings.

I ask this as an AR repair novice.

 

[MistWolf]

The gun has always been spotty with locking back on the last round but now with the BA barrel it short strokes so bad that with all but the hottest loads you have to cycle the bolt by hand with each round as it doesn't cycle far enough to pick up a round....
...Is there something I'm missing in the gas system? Something to check in the bolt carrier?

-The rifle has always exhibited problems with short stroking with multiple barrels.
-The rifle buffer is the right weight. Well, it was before being disassembled.
-Gas tube isn't blocked.
-Chances of the gas block being misaligned in four different barrels is very small. Unless the same gas block is being used over an over again and the gas block is out of spec.
-I believe a .090" gas port is a little on the large side for a 20" barrel. I think the norm is .086", but I could be wrong.

Therefore, I'd say your problem is the BCG. Try a different BCG in the upper and see what happens. Before you do, reassemble the buffer. If you don't have a spare BCG, get one. Having one on hand saves a lot of time and trouble.

I have no experience with side charger BCGs. Is it possible that the hole for the charging handle intrudes into the expansion chamber and is creating a leak path?

I assume that your AR has a rifle length RE, not an A5 or carbine? I'm asking because the first question tech support should be asking is "Are you certain your device is plugged in and switched on?" Let's eliminate the obvious and start with the simplest solutions.

 

[someguy2800]

-The rifle has always exhibited problems with short stroking with multiple barrels.
-The rifle buffer is the right weight. Well, it was before being disassembled.
-Gas tube isn't blocked.
-Chances of the gas block being misaligned in four different barrels is very small. Unless the same gas block is being used over an over again and the gas block is out of spec.
-I believe a .090" gas port is a little on the large side for a 20" barrel. I think the norm is .086", but I could be wrong.

Therefore, I'd say your problem is the BCG. Try a different BCG in the upper and see what happens. Before you do, reassemble the buffer. If you don't have a spare BCG, get one. Having one on hand saves a lot of time and trouble.

I have no experience with side charger BCGs. Is it possible that the hole for the charging handle intrudes into the expansion chamber and is creating a leak path?

I assume that your AR has a rifle length RE, not an A5 or carbine? I'm asking because the first question tech support should be asking is "Are you certain your device is plugged in and switched on?" Let's eliminate the obvious and start with the simplest solutions.

I have checked and the gas port in the barrel is fully within the opening of the gas block. You can see it visually with the gas block off that there is a soot ring totally around the gas port. I don't have another bolt carrier group to try and wasn't planning on getting one. I'm not going to be an AR collector, I have one lower and two uppers and no plans to get any more. I will see if I can borrow one to try from someone. The side charging handle screws in ahead of the expansion chamber, roughly inline with the cam pin (it does not protrude through, I checked). The receiver extension is the one that came with the A2 stock kit. It came as a package with the stock, buffer tube, spring, and buffer.

 

[MistWolf]

You will find your problem is the carrier. Either the gas key leaks, the rings are out of spec or the expansion chamber is out of spec. I have a carrier that the expansion chamber leaks excessively. I know it's the expansion chamber because new rings didn't fix the problem and gas key isn't leaking.

Having a spare carrier has nothing to do with being an AR collector. It's a spare part to assist in troubleshooting and to keep your AR running when situations like this happen.

You said you have two uppers? You can swap the BCG from one to the other. Does your side charger upper allow the use of a standard charging handle? If not, remove the side charger handle from the carrier in question and run it in the other upper.

Another option is to swap bolts to check the gas rings. Headspace won't be a problem.

 

[someguy2800]

I 'm planning on using the same bolt carrier with both uppers. The 2nd is 300 blk that I just got and havn't shot yet. The side charging carrier is just drilled and tapped for a handle on the side so I'm pretty sure a standard carrier from another gun would work. The gas rings are a real tight fit in the carrier. I don't know if they can be to tight? It takes about 2 lbs of pressure to push the bolt in with no cam pin. That is if I pull the bolt out it takes a 2 lb weight sitting on it to push it back in. Its just really strange to me how much this issue has changed from one barrel to another. Its never been even close to this bad with any of the other barrels using the same gas block and tube. I guess for now I will just leave the light buffer in it and if something changes I will experiment further.

 

[LoonWulf]

You ever try blowing thru the gas tube?

Ive read that Its generally advisable to keep bolts and barrels as a set, they ware together with use and moving bolt from barrel to barrel can cause that ware to be inconsistent. Ive always built full uppers so ive never gotten to find out first hand if thats true. Buying a second bcg isnt much of an investment tho.

Also a note on the bca side charger and the blackout, i found that the bolt would drop the short case before getting it out of the ejection port sometimes. The start of port seemed a bit farther back than on my other uppers so that maybe something to watch.

 

[RussellC]

The gas tube and gas block line up well. I don't have any frame of reference of how tight the fitment between the gas key and the tube should be. When you slide the two together there is a tiny bit of movement between the two. Probably a couple thousandths clearance Everything move very smoothly, no hangups at all operating it by hand.



Never heard of the drop test, had to google it. With the carrier group standing on end it takes about 2 lbs of force on the carrier to cam the bolt. I checked and the gas key is tight. Took the bolt out and the rings were staggered.



Its a rifle length gas system on a 20" barrel.

One small thing I noticed when I was taking it back apart to check tonight, there is a small amount of gas leakage between the gas tube and the gas block. There is some soot around the gas tube where it goes into the block and there is a tiny bit of soot blown onto the top of the barrel. I'm assuming this is normal since none of these parts have any positive seal on them. I never would have noticed it if not for being a stainless barrel.

I had similar leaking around the gas block tube as you discuss. At first, like yours it wouldnt lock back, cycle, pick up next shell...nothing.

Slowly, it got to where it would cycle most times but not always. It still would not lock back.

Then, it locked back a couple times. This is when I noticed the leaking signs. It seemed to be getting better, but I replaced the gas block and tube. That cured the problem completely.
Most frustrating.

Russellc

 

[someguy2800]

You ever try blowing thru the gas tube?

Ive read that Its generally advisable to keep bolts and barrels as a set, they ware together with use and moving bolt from barrel to barrel can cause that ware to be inconsistent. Ive always built full uppers so ive never gotten to find out first hand if thats true. Buying a second bcg isnt much of an investment tho.

Also a note on the bca side charger and the blackout, i found that the bolt would drop the short case before getting it out of the ejection port sometimes. The start of port seemed a bit farther back than on my other uppers so that maybe something to watch.

Yep I did blow through the gas tube and also stuck a 3/32 aluminum tig filler rod down it. Thanks for the heads up on the potential issues. I will keep an eye out for a bca carrier when they get back in stock.

 

[someguy2800]

I had similar leaking around the gas block tube as you discuss. At first, like yours it wouldnt lock back, cycle, pick up next shell...nothing.

Slowly, it got to where it would cycle most times but not always. It still would not lock back.

Then, it locked back a couple times. This is when I noticed the leaking signs. It seemed to be getting better, but I replaced the gas block and tube. That cured the problem completely.
Most frustrating.

Russellc

I was thinking that it might get better as the leaks fill up with carbon fouling. I do have another gas block, I should try swapping them.

 

[MistWolf]

Ive read that Its generally advisable to keep bolts and barrels as a set, they ware together with use and moving bolt from barrel to barrel can cause that ware to be inconsistent.

This is generally true. It won't affect reliability much, but it could have an impact on precision when swapping well used bolts around.

...the bolt would drop the short case before getting it out of the ejection port sometimes.

Try a new extractor and a Colt or Sprinco M4 extractor spring.

 

[LoonWulf]

This is generally true. It won't affect reliability much, but it could have an impact on precision when swapping well used bolts around.


Try a new extractor and a Colt or Sprinco M4 extractor spring.

I replaced the extractor and spring and even tried a second bolt to no avail. The ejector could kick the case free before it left the gun.

 

[G.barnes]

What brand were the other 3 barrels. Psa has bcg on sale for super cheap right now. I'd pick one up and give it a try. Worst case use it in your new upper. It sounds like your other barrels were probably overgassed so it masked the problem. BA barrel are conservatively ported. What brand is your bcg. Don't take this the wrong way, but from your parts list you've used the cheapest parts you could find and something doesn't work. Probably because one or two parts are out of spec.

 

[RussellC]

I was thinking that it might get better as the leaks fill up with carbon fouling. I do have another gas block, I should try swapping them.

That is how I thought it would go with mine. It was slowly "getting better" and it was a fairly inexpensive adjustable block...but all my other builds have worked fine right out of the box. Something about it bothered me, so I replaced it.

That said, I do feel if I kept using it, it would have worked? Likely.

Also, all my builds happen to have BA barrels, and those that didnt originally do now. Their port sizes are given, and they have been standard size on the ones I have, but they do have various barrels.

Also, the old block didnt work even full open. Taking it and the tube off the barrel, holding the barrel end shut and blowing on the bolt end of the tube resulted in condensation bubbles forming where tube goes into block. I really dont think, (at least with my particular BA barrels) that the port was too small. I would strongly advise against drilling it out, which even if done correctly by a gunsmith, can also result in over gas situation.

I think you got a leaky one, standard blocks are not expensive...if that doesnt fix it and everything is lining up in term of ports on barrel and block, there isnt a whole lot left.

I have seen this occur to some degree with the buffer and spring not matched to the barrel type/length, carbine, A2, rifle length, medium and so forth, but not causing problems to the degree mine did.

I also tried the removing some buffer weight with some improvement, but full fix was a new block. They arent that expensive, and if you are building them, they wont go to waste. That said, my problematic adjustable sits in a parts box!

Russellc

 

[RussellC]

Just checked your list again, I had not noticed the side charger upper, have no experience with them.

I also, as another poster noted, spring/stock/buffer were inexpensive...not that they cant work, but I usually dont scrimp here and go for a known quality, many fit this description, I like Spikes. For standard type gas blocks, I like geiselle or comparable. Hard to avoid foriegn made parts, especially at low price points but I like USA made parts in AR rifles.

Russellc