Short stroking problem with new ar 15

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[ARCHIVED THREAD] - Short stroking problem with new ar 15

Posted: 1/3/2012 6:26:21 PM EDT

Hey all. This is my first post to ar15.com so bare with me. I just put together my first ar and I am having a problem with it short stroking every time. It cycled properly for one shot. With only one round in the magazine, once fired the bolt does not go far enough back to catch the bolt catch. My setup is a 20" dpms bull barrel, LMT bolt carrier assembly, daniel defense upper, RRA lower with a collapsable stock with normal carbine buffer. I have cleaned and lubricated the gun several times and it will only eject the shell but never load another in. I have removed the gas tube and gas block and do not see any abnormal wear to the tube. I have tried slightly moving the gas block and still the same thing. I have checked the buffer and spring and made sure the bolt carrier slides into the buffer tube without any problems. The gas keys are in proper position. I have shot about 35 rounds through the gun using mostly 55g PMC ammo and a few 40g fiocchi. At this point I am not sure what is next to try. Please any input would be helpful. Thanks.

Posted: 1/3/2012 8:27:37 PM EDT

[#1]

PMC is likely the weakest ammo you could find. I would try some higher power ammo, after a few boxes it may run on the weaker stuff.

Make sure that your bolt can retract far enough to the rear. It should be able to be retracted about 3/32" to 1/4" behind the bolt catch when pulled all the way to the rear with the charging handle. Make sure the gas key is tight and you don't see any evidence of other gas leaks or gas block misalignment. Might also check the lower to make sure nothing is preventing the hammer from retracting fully, so that it's not dragging excessively on the bottom of the bolt.

If you don't find any other issues, make sure you have lubrication on the inside of the bolt/bolt carrier area (suggestion would be a relatively heavy coat of a lighter weight oil), try some higher power loads, and see if you get function.

Posted: 1/4/2012 8:15:19 PM EDT

[#2]

Short stroke - Rifle gas system generates approximately 13,000 psi, Carbine generates 26,000 PSI you need balance for the system for it to work properly, Carbine spring and buffer are designed to handle the higher pressure. May sound crazy, but Check for gas leak, and reduce buffer weight, or lighter bolt carrier, or adjustable gas block, or lighter spring or a combination of things. Search buffer weights or ejection chart .

Posted: 1/5/2012 4:42:13 PM EDT

[#3]

you have a rifle length gas system? you need a rifle buffer and spring. I always break in my AR's and sight-in with 5.56 ammo (M193) for the first couple hundred rounds, the after that i switch to whatever i can get cheap.

Posted: 1/5/2012 4:57:44 PM EDT

[#4]

Quoted:

you have a rifle length gas system? you need a rifle buffer and spring...

That's completely inaccurate.

A 20 rifle is perfectly capable of running with a collapsible stock.

Posted: 1/6/2012 3:14:12 AM EDT

[#5]

Quoted:

Quoted:

you have a rifle length gas system? you need a rifle buffer and spring...

That's completely inaccurate.

A 20 rifle is perfectly capable of running with a collapsible stock.

Ditto, buffer spring and buffer has all to do with the internal length of the receiver extension tube, and not the upper receiver and it's barrel length.

If you looked at how everything works tacked thread, although different barrel lengths will have different pressures at the gas port, the barrel gas ports are sized accordingly on each length barrels to send the correct amount of pressure dwell back to the carrier to cycle the rifle correctly.

Posted: 1/6/2012 4:39:51 AM EDT

[#6]

Thanks for the input. I'm hopefully gonna get together with one of my friends who has an ar and try swapping some parts this weekend to maybe narrow the issue down. Ill try some hotter loads first and see what happens.

Posted: 1/6/2012 8:48:06 AM EDT

[#7]

On the telescoping stock, pull the buffer and spring, and make sure that you have the correct buffer and spring is place. Carbine buffer is pretty easy to spot if you have the wrong one in (will be much longer than a carbine buffer for a telescoping stock, but on the recoil spring , it should be around 10.5" , and not a standard fixed spring length of 11.75" long.

Or if you just want to get lazy and not pull the buffer and spring, pull all the way back on the charging handle and look to make sure that the face of the bolt has retracted back past the mag catch,but the bolt face did not retract back past the back of the ejection port window, and on the last inch'ish of the charging handle pull, the charging handle tension did not load up from a way too long spring just coil bind'd on itself at the last of the pull.

Next, pull the bolt off the carrier, and dry fit the carrier alone in the upper receiver. Make sure that the key is not binding in the upper slot, and the gas tube is correctly aligned with the key (last inch'ish as the face of the carrier goes to touch the face of the barrel extension.

While you have the carrier in hand, put an allen wrench on the key bolts to make sure that they are tight (should be 37 in lbs torqued).

Gas tube, uses a piece of rubber tube to blow down the tube from within the receiver to check air flow of the tube out the bore. Granted that you may have the gas block passage set correctly over the barrel gas port, but if gas tube was not installed correctly in the block (pin not through the channel in the gas tube, but in front of the tube instead, or the tube installed upside down with its opening pointing up), your not going to get flow.

And as for ammo, try some Win white box ammo. As stated, the PMC can be a tad under powder, and in regards to the 40's, complete wrong powder for the platform used in them to get the correct gas pressure dwell to cycle the B/C correctly (burn dwell time to fast on the gun powder being used, which causes less pressure at the gas port than needed to correctly cycle the action).

Posted: 1/6/2012 11:12:25 AM EDT

[#8]

Quoted:

Quoted:

you have a rifle length gas system? you need a rifle buffer and spring...

That's completely inaccurate.

A 20 rifle is perfectly capable of running with a collapsible stock.

Ditto, buffer spring and buffer has all to do with the internal length of the receiver extension tube, and not the upper receiver and it's barrel length.

If you looked at how everything works tacked thread, although different barrel lengths will have different pressures at the gas port, the barrel gas ports are sized accordingly on each length barrels to send the correct amount of pressure dwell back to the carrier to cycle the rifle correctly.

so you are telling me that the gas system length has nothing to do with the buffer spring and buffer weight?

Posted: 1/6/2012 2:02:38 PM EDT

[#9]

Quoted:

... so you are telling me that the gas system length has nothing to do with the buffer spring and buffer weight?

No, that's not what's been said.

Posted: 1/6/2012 4:00:39 PM EDT

[#10]

Quoted:

Quoted:

... so you are telling me that the gas system length has nothing to do with the buffer spring and buffer weight?

No, that's not what's been said.

OK, because in my original statement I clearly asked if he had a rifle length gas system. Then i said something about a rifle spring and buffer and somehow that got turned into a me getting TOLD I was wrong. Now that I clarifed, I'm still wrong? That is what has been said.

Posted: 1/6/2012 4:04:58 PM EDT

[#11]

Quoted:

Quoted:

... so you are telling me that the gas system length has nothing to do with the buffer spring and buffer weight?

No, that's not what's been said.

OK, because in my original statement I clearly asked if he had a rifle length gas system. Then i said something about a rifle spring and buffer and somehow that got turned into a me getting TOLD I was wrong. Now that I clarifed, I'm still wrong? That is what has been said.

At this point, you've made it clear that you don't have a basic understanding of the topic.

ETA: Please don't take that as an insult. Do you understand the differences between a rifle/A2 buffer system and a carbine buffer system?

Posted: 1/6/2012 4:13:10 PM EDT

[#12]

Quoted:

Quoted:

... so you are telling me that the gas system length has nothing to do with the buffer spring and buffer weight?

No, that's not what's been said.

OK, because in my original statement I clearly asked if he had a rifle length gas system. Then i said something about a rifle spring and buffer and somehow that got turned into a me getting TOLD I was wrong. Now that I clarifed, I'm still wrong? That is what has been said.

At this point, you've made it clear that you don't have a basic understanding of the topic.

Thanks for the education.

Posted: 1/6/2012 4:14:06 PM EDT

[#13]

See my ETA comment above.

Posted: 1/6/2012 4:31:22 PM EDT

[#14]

Quoted:

See my ETA comment above.

YES I understand. I am trying to help here and it feel like you two are all over me. I think you are missunderstanding what i am trying to say. I asked what gas system he had. he never said. To my understanding, a carbine spring and buffer will cause short stroking if used in the longer gas systems. We still dont know what gas system he has, all we know is he has a collapsable stock and a carbine buffer and I'm fully aware that the stock choice has nothing to do with barrel length.

Posted: 1/6/2012 8:09:22 PM EDT

[#15]

Quoted:
so you are telling me that the gas system length has nothing to do with the buffer spring and buffer weight?

Correct!

Posted: 1/6/2012 9:30:21 PM EDT

[#16]

Quoted:

Quoted:

See my ETA comment above.

YES I understand. I am trying to help here and it feel like you two are all over me.

Honestly, no one is "all over you".

I think you are missunderstanding what i am trying to say.

That's possible ... my apologies if so.

I asked what gas system he had. he never said.

True, but he did state that he's using a 20" bull bbl, and it's a pretty safe bet that it's got a rifle length gas system.

To my understanding, a carbine spring and buffer will cause short stroking if used in the longer gas systems.

This is really the crux of the matter ... if using a collapsible stock, then there are several variables in terms of different buffer weights and springs - but - to make a blanket statement that a carbine buffer system won't work well, and what's necessary for longer gas systems is a rifle buffer system, simply isn't accurate.

We still dont know what gas system he has, all we know is he has a collapsable stock and a carbine buffer and I'm fully aware that the stock choice has nothing to do with barrel length.

Correct, the stock itself is not really a factor.