Wilson Combat full auto bolt in AR15 not cycling?

[gulada97]

I put together an AR15 using an Aero Precision lower and Armalite upper. I got a "full auto" Wilson Combat bolt that somehow will not cycle properly. It'll shoot once or twice and then jam. All my other "semi auto" bolts work just fine. I've tried it in 3 different ARs and it just won't work.
I'm using a rifle length gas system/18" barrel and Aero Precision carbine buffer tube.
Does it need a shorter gas system to work properly?

 

[longshot2000]

The problem has nothing to do with full auto or semi-auto carrier. They will both function the same. With different brands, it is always hard to find the culprit. You did not mention the barrel maker. 18" barrel is a little more sensitive than 20" with rifle length gas system. You get more pressure in the tube for a shorter period of time. Most 18" barrels with rifle length system will also show early port erosion, but that is after thousands of rounds.

the problem and the fix is going to be with the buffer and/or the buffer tube.

You see, the physics of an 18" barrel give you less time for the gas to cycle through to the carrier before the bullet exists, so you get a shorter "dwell time" than with a 20" barrel. So, a little more kick for a shorter period of time.

I would have two suggestions. The first is: I have never used a carbine buffer tube with a rifle length gas system. I am sure others have. Normally, a rifle lenght gas system is fit with a rifle length buffer tube. If you dont' mind moving to a rifle buffer and tube, that would be my first recommendation.

Your easiset choice, if you choose to keep a carbine buffer tube, is to alter the buffer weights. I cannot tell from your description if the buffer is not going back far enough, or if is goiing back far enough, but returning too fast. You did not mention which buffer you had. Again, while I am a barrel nerd, I have just never tried to use a carbine tube with a rifle lenght gas system. If you have an H2 or H3 buffer, it is probably too heavy. If you have an H1 it also might be too heavy. If you have a standard carbine buffer, it might be too light for the gas presssure in a rifle gas system. Either way, you can replace one steel weight with a tungsten weight to make it heavier and try that, or replace one steel weight with an aluminum weight and make it lighter.

 

[gulada97]

Barrel came with the Armalite upper : not much on their website except that it's "HB, match grade, stainless". The buffer (99.99%) sure is 3 ozs. That is a good point however, I remember reading about buffer weights. The gun was built as a 16" lightweight originally and I swapped the upper for the 18" competition one from Armalite. Buffer weights are cheap enough, I'll try a heavier one!
Thanks!

 

[longshot2000]

buffer weights are dirt cheap. look: AR15 Carbine tungsten buffer weights Or, get a variable weight carbine buffer set

If you changed from 16" to 18" that would make sense. You can try changing out the buffers or weights, or put a rifle length buffer tube and stock on the gun. What you have is not optimized to work together. AR's are somewhat like lego blocks, but you have to start with the proper barrel setup and work from there.

 

[drewthebrave]

Can you describe the type of jam? What is the ejection pattern of your brass (when it cycles & ejects)?

There is no such thing as a "Full Auto Bolt". A "Full Auto Carrier" is only marginally heavier than a "Semi Auto Carrier", so it shouldn't make much difference either way.

I would recommend swapping the bolts between the carriers and see if the problem follows the bolt or if the problem follows the carrier.

 

[Near miss]

Can you describe the type of jam? What is the ejection pattern of your brass (when it cycles & ejects)?

There is no such thing as a "Full Auto Bolt". A "Full Auto Carrier" is only marginally heavier than a "Semi Auto Carrier", so it shouldn't make much difference either way.

I would recommend swapping the bolts between the carriers and see if the problem follows the bolt or if the problem follows the carrier.

Sorry little lost with how ARs function, but can I ask why is the full auto bolt heavier in the back? To reduce fire rate?

 

[drewthebrave]

Sorry little lost with how ARs function, but can I ask why is the full auto bolt heavier in the back? To reduce fire rate?

Correct. The extra mass slows down the cyclic rate to improve reliability during full auto fire. For recreational use, it's less critical, but can help tune the cyclic rate for more reliable operation.

 

[jbailey]

hello @gulada97
this chart might help you. Maybe test and see where the brass that does eject lands. If its 4:30 or later, you know you have to increase the gas and/or lighten up the mass (BCG, buffer, spring).

 

[Stickman]

What do you mean by "jam"?

 

[gulada97]

What do you mean by "jam"?

it'll fire 2-3 rounds then get stuck while ejecting the brass. So I have to pull the charging handle all the way back to remove the brass. Kinda weird, it's like it pulls the brass out of the chamber maybe 3/4" and then just gets stuck.

 

[gulada97]

hello @gulada97
this chart might help you. Maybe test and see where the brass that does eject lands. If its 4:30 or later, you know you have to increase the gas and/or lighten up the mass (BCG, buffer, spring).

View attachment 7464419

This is great, thank you!

 

[Gregor.Samsa]

it'll fire 2-3 rounds then get stuck while ejecting the brass. So I have to pull the charging handle all the way back to remove the brass. Kinda weird, it's like it pulls the brass out of the chamber maybe 3/4" and then just gets stuck.

I had a very similar situation for a while with my 18” AR. Would fire a shot or two then would have to pull the charging handle to cycle the next round. I chased my tail trying to fix this for quite a while. I swapped out buffer tubes etc. ultimately, I finally diagnosed and rectified the issue by swapping the trigger. I had a Jewell adjustable trigger which felt absolutely amazing, but I swapped it for a LaRue mbt2s and now the rifle functions perfectly. I don’t know exactly why but it had to do with the geometry of the interface with the BCG. Just throwing this out there if swapping the buffer spring etc. fail to remedy your issues. I’m the furthest thing from an expert so my advice may bet worth it’s cost. Good luck.

 

[oldfatguy]

Correct. The extra mass slows down the cyclic rate to improve reliability during full auto fire. For recreational use, it's less critical, but can help tune the cyclic rate for more reliable operation.

The extra material does add a little mass. But it’s real function is to trip the secondary sear during FA fire.

OFG

 

[gulada97]

I had a very similar situation for a while with my 18” AR. Would fire a shot or two then would have to pull the charging handle to cycle the next round. I chased my tail trying to fix this for quite a while. I swapped out buffer tubes etc. ultimately, I finally diagnosed and rectified the issue by swapping the trigger. I had a Jewell adjustable trigger which felt absolutely amazing, but I swapped it for a LaRue mbt2s and now the rifle functions perfectly. I don’t know exactly why but it had to do with the geometry of the interface with the BCG. Just throwing this out there if swapping the buffer spring etc. fail to remedy your issues. I’m the furthest thing from an expert so my advice may bet worth it’s cost. Good luck.

Glad you chipped in. I did try the bolt in another AR with Elftmann trigger vs Timney and it still failed. Granted, it's got the same buffer weight buy a shorter barrel and carbine gas system.

 

[Yondering]

Glad you chipped in. I did try the bolt in another AR with Elftmann trigger vs Timney and it still failed. Granted, it's got the same buffer weight buy a shorter barrel and carbine gas system.

I kind of feel bad that you've gotten so much bad advice in this thread from people who don't know how to troubleshoot a problem.

You already said you've swapped that bcg into 3 different rifles and the problem follows the bcg, right? That doesn't indicate a need to chase buffer weights, and unlikely to be a trigger issue either. It really sounds like a problem specific to the bcg somehow.

The two common issues with bolt carrier groups that can cause cycling problems are the gas key on the carrier, and the gas rings on the bolt.

Start with the gas key. Is it loose? Are there any signs of gas/carbon being blown out where the key mates to the carrier? That'll do it, and is the reason gas key bolts get torqued and staked. The effects of a loose gas key can be similar to the symptoms you've described as well.
If the gas key is partially plugged up that can cause an issue too, but it's less common.

Worn out (or just plain bad) gas rings are the other likely culprit. With the bcg assembled, extend the bolt and stand the whole assembly vertically on the bolt. It shouldn't collapse; if it does your gas rings don't have the tension they should, and may not be sealing well. (You'll hear some conflicting opinions on this gas ring test, and it's not the final word in gas ring performance, but can help for troubleshooting.)

A couple other items to check, although less likely are:
- Broken cam pin?
- Severe wear of the bolt tail or matching hole in the carrier (this area has the same function as the gas rings, it's the other end of that pressure chamber)
- Unusual/strange wear patterns in your upper receiver (whichever one you've been using this carrier in). That can indicate something strange going on.

Post some pics of the stripped bcg parts if you can't find anything obvious.

 

[S197]

it'll fire 2-3 rounds then get stuck while ejecting the brass. So I have to pull the charging handle all the way back to remove the brass. Kinda weird, it's like it pulls the brass out of the chamber maybe 3/4" and then just gets stuck.

when the carrier fails to cycle, dose it take a lot of force to clear the jam? have you tried hand cycling some rounds til failure? also post some pics of your rifle, bcg.

 

[UpSideDown]

I agree there is terrible advice in this thread. Rifle and carbine buffers function identically, there is zero reason to switch to a rifle system.

Have you headspaced the chamber with this bolt?? if you are having pressure issues that can affect extraction

 

[gulada97]

Thanks gents, will post pics tomorrow. I currently have

Could also be an improper gas port size in the barrel. I have seen that before. Will the BCG would in other rifles?

negative. It won't work in my armalite or ruger. Rifle gas and carbine gas.

 

[c ward]

The gas system length has nothing to do with what bolt carrier is used . M4 and M16 have the same bolt carriers . The full auto carrier does 2 things, add weight to slow the cyclic rate slightly, and trip the auto sear with the bottom of the carrier that isn't present on a semi auto only carrier . A full auto bolt carrier will function normally in a SA rifle .

Yonderling above is on the path that I would take since you say you have moved the bolt carrier group to different uppers and the issue follows the bolt .

 

[gulada97]

I kind of feel bad that you've gotten so much bad advice in this thread from people who don't know how to troubleshoot a problem.

You already said you've swapped that bcg into 3 different rifles and the problem follows the bcg, right? That doesn't indicate a need to chase buffer weights, and unlikely to be a trigger issue either. It really sounds like a problem specific to the bcg somehow.

The two common issues with bolt carrier groups that can cause cycling problems are the gas key on the carrier, and the gas rings on the bolt.

Start with the gas key. Is it loose? Are there any signs of gas/carbon being blown out where the key mates to the carrier? That'll do it, and is the reason gas key bolts get torqued and staked. The effects of a loose gas key can be similar to the symptoms you've described as well.
If the gas key is partially plugged up that can cause an issue too, but it's less common.

Worn out (or just plain bad) gas rings are the other likely culprit. With the bcg assembled, extend the bolt and stand the whole assembly vertically on the bolt. It shouldn't collapse; if it does your gas rings don't have the tension they should, and may not be sealing well. (You'll hear some conflicting opinions on this gas ring test, and it's not the final word in gas ring performance, but can help for troubleshooting.)

A couple other items to check, although less likely are:
- Broken cam pin?
- Severe wear of the bolt tail or matching hole in the carrier (this area has the same function as the gas rings, it's the other end of that pressure chamber)
- Unusual/strange wear patterns in your upper receiver (whichever one you've been using this carrier in). That can indicate something strange going on.

Post some pics of the stripped bcg parts if you can't find anything obvious.

Thank you sir, I'll look into that!