Suddenly shortstroking, Need help

[George Post]

I have a fairly new upper on my rifle with about 250 rounds through it. It was working very well. But at the same time my Bushmaster two stage trigger was turning into a single stage trigger. I removed the hammer and greased the sear and hammer. The trigger seemed to be working fine now. In fact before I lubed it, it had a hard time setting off Wolf .233 primers, now that seems to be cured.

Last time I shot the rifle it was 30 degrees out and today it was 65 degrees, I made no other changes other then lubing the sear and hammer.

At the range I lubed it, changed mags twice( two of the mags were new, one was a known good working mag.) When that didn't fix it I took out the bolt alined the gas rings so that the openings were one third apart, cleaned and reassembled. still no joy.

The rifle is a Bushmaster lower with a bushmaster two stage trigger, carbine length buffer tube, 20 barrel.

What steps should I take to trouble shoot this?

Thanks

George

[EkuJustice]

Have you checked the gas key on the rifle to make sure it is tight and properly staked? What kind of gas block did you have on the rifle? If it is not a pinned on one, have you checked to make sure it isn't bumped off to the side a little bit.

[jobob]

Any change in ammo, or anything else? Wolf ammo? My 20" Bushy shortstrokes on Wolf, but works well with good ammo.

[Alaskapopo]

You have a gas problem. Start at the gas block and make sure its tight if it is one that uses allen screws. Also check for gas blow by on barrel in that area that would indicated a gas leak. Then got to the carrier key on the bolt carrier make sure its tight. Then check the gas rings.

Pat

[Bryan 45]

Craziest gas problem I've seen was a spent primer (from Wolf ammo) getting lodged into the gas key. It's just the right size to fit back into the key far enough that you can't see it. The rifle doesn't short-stroke though, it just flat doesn't move.

[mpeltier]

Wolf ammo??????

[Precision40]

Carbine length buffer tube on a 20" upper? Isn't that a no-no?

[wsimpso1]

My first question is: Round in the chamber; empty mag in the well; fire the round - Does it lock open? It should lock open everytime.

If it does not, then you have a gas problem. The above responses cover things pretty well.

As to buffers, springs, and tubes, as long as you use whole sets (all carbine type or all rifle type) it shouldn't matter a bit which length top end you put on it. Now if you put a carbine buffer in a rifle length buffer tube, that can destroy stuff.

Billski

[Nik Habicht]

Carbine length buffer tube on a 20" upper? Isn't that a no-no?

Not if it's a carbine length stock.....

Buffer tube lengths/buffers and stocks need to match --- but you can run any type of upper with the right combo of parts. The upper doesn't care if it's on a rifle length lower or a carbine stocked lower, as long as the parts match up....

[Nik Habicht]

I have a fairly new upper on my rifle with about 250 rounds through it. It was working very well. But at the same time my Bushmaster two stage trigger was turning into a single stage trigger. I removed the hammer and greased the sear and hammer. The trigger seemed to be working fine now. In fact before I lubed it, it had a hard time setting off Wolf .233 primers, now that seems to be cured.

Last time I shot the rifle it was 30 degrees out and today it was 65 degrees, I made no other changes other then lubing the sear and hammer.

At the range I lubed it, changed mags twice( two of the mags were new, one was a known good working mag.) When that didn't fix it I took out the bolt alined the gas rings so that the openings were one third apart, cleaned and reassembled. still no joy.

The rifle is a Bushmaster lower with a bushmaster two stage trigger, carbine length buffer tube, 20 barrel.

What steps should I take to trouble shoot this?

Thanks

George

George,

any left over parts after dis/reassembly? :devil:

[CocoBolo]

My first question is: Round in the chamber; empty mag in the well; fire the round - Does it lock open? It should lock open everytime.

If it does not, then you have a gas problem. The above responses cover things pretty well.

As to buffers, springs, and tubes, as long as you use whole sets (all carbine type or all rifle type) it shouldn't matter a bit which length top end you put on it. Now if you put a carbine buffer in a rifle length buffer tube, that can destroy stuff.

Billski

My Bushmaster is doing essentially the same thing. It runs one stage and then it starts burping, farting and falling. I use PMC Ammo, I had my smith clean and ream every pore in the thing, still don't run. Tomorrow I'm taking a range day to do Exorcism on it. I will test the bolt lock first thing. I started having issues after the JP trigger springs were installed.

[George Post]

A lot of great ideas, thanks guys.

Gas key looks and feels tight, the gas block looks straight, but kinda hard to check as my super smart alter ego put a 16 inch carbon fiber tube over the gas block.

My ammo is loaded with 24.1 gn. of AA2230 with Wolf primers not Wolf ammo. Chronos at 2975 fps. with 55 gr Hornadys. The last time I shot this rifle it functioned just fine, locking back on an empty mag. Now it won't even move the next round at all.

I'm going to try my spare bolt and carrier on Sunday, then if that is not the problem, attack the gas block then the gas tube.

Thinking back to when I assembled this upper I think I made a mistake, and shaved the mushroom off the end of gas tube. I thought it was a manufacturing mistake. But it ran just fine until yesterday.

Thanks for the help, I'll update you Sunday and see where to go from there.

George

[George Post]

Nik,

After I get this MultiQuote thing to work, I'll come back with a some kind of wise-guy remark.

See you on the range

George

[jobob]

Your modification of the end of the gas tube may have something to do with it.

Also, your gas key may seem tight when you check it cold, but loosen as it heats up. Does the problem get worse as the gun heats up, or the same from the first shot fired.

You may have painted yourself into a corner by installing the long carbon fiber tube over your gas block. I believe those hand guards are glued to the barrel nut, is that right? If so then you can't get to the gas block without destroying, or heavily modifying, your hand guard.

[George Post]

Update: I swapped out the carrier and bolt and it ran real well.

On closer inspection one of the bolts holding the gas key on was sheared off, but was held firmly by the stakeing.

Thanks to everyone for your help.

George

[wsimpso1]

Gas key bolts do not shear off for no reason. Usual issue for one to be failed is it was tightened almost to failure at assembly. Usual issue for two is that the clamp load was inadequate and the key was sliding.

Fix that carrier by removing and tossing what is left of the bolts and carrier key. Procure new carrier key and bolts. Clean threads, check that new bolts run easily all the way down. If not, run the correct tap in the holes to true them up. Check that the carrier key fits. I like to use a bit of fine lapping compound to dress them together. Clean, assemble, torque the fasteners. Success depends upon it being a solid fit, good torque on the bolts, and a way to keep them in place. I prefer the Loctite solution - I have never had one loosen until I wanted it to, but then I degrease thoroughly prior to using the stuff. If staking, lube the threads with oil before torqueing.

Billski