AR15 short stroke issue

[djgaloot]

I am new to the AR scene and recently purchased a used but fairly new rifle with an 18" barrel with rifle length porting. The BCG is a full auto style and the buffer/spring assembly is a bit heavier than stock (I believe). It will cycle with some (but not all) heavier, stronger ammo but will not cycle with rounds such such as Federal 55 gr. I believe it is short stroking because it will fire and eject but not pick up the next round and will not hold the bolt open after the last round is fired. i need to manually pull the charge handle between shots. My question is what is the proper relationship between gas port length, Carrier group mass and buffer spring mass and stiffness. It seems I may be out of balance or I need a different combination to shoot lighter ammo. I swapped the BCG and buffer into my brothers 16" Stag and it cycled flawlessly with the federal ammo. I have stripped it and cleaned it, etc. Any thoughts?
Thanks

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[MilesTeg]

Have you checked to see if you gas block is aligned properly or fouled. Heavier loads would cause more pressure and overcome the problem that lighter rounds can't. More gas following the bullet than being channeled into the gas tube.

 

[djgaloot]

I have not pulled off the gas block yet. what is the proper way to inspect/clean. Should I just use a bore brush down the tube and blow compressed air?

 

[jdub75]

is there an imprint on the end of the buffer? It may be an H or H2 or H3 buffer in there. If its an H2 or H3 swap it for a standard weight buffer. This can cause short stroking, assuming the gas tube isn't leaking/blocked up.

Forum on buffers:
http://www.ar15.com/forums/t_3_118/416283_Buffer_weights___.html

 

[jdub75]

It can be nearly impossible to remove a gas tube without damaging it. Be prepared for frustration if you try, and maybe have a spare around!

 

[MilesTeg]

Gunsmith is your friend. But if you are adventurous you can Youtube some videos on how to remove and install the gas block. I would try squirting some powder blaster down the tube and then follow up with compressed air. If that doesn't work then time for a smith or DIY. Was this rifle bought new, used, or home built?

 

[djgaloot]

It is not marked but I weighed it on an accurate postal scale at 3.0 oz.
it is a Palmetto State Armory buffer.

 

[djgaloot]

It was built by a fellow NWFA member. nice rifle that has a brand new lower and low round count upper. I was told it did not like smaller ammo and knew it when I bought it so no animosity whatsoever. It shoots green tips fine. I would like to try and get it to be more versatile.

 

[djgaloot]

It can be nearly impossible to remove a gas tube without damaging it. Be prepared for frustration if you try, and maybe have a spare around!

I will research this a bit before diving in. Thanks.

 

[cityrice]

I recently had a similar issue, it turned out to be the gasblock hole for the gas tube was a loose fit and created a leak.
Not enough pressure to push the bolt completly back.
Just enough to eject a round but not enough to chamber a new one.

I bought a clamp on gas block Daniel Defense and a new gas tube from midway and replaced it works perfectly.

 

[DoubleTapDrew]

Where is it ejecting the spent cases? 3oz is a lightweight buffer so it may be cycling faster than the magazine spring can feed the next round in.
Did you try swapping your brother's buffer into yours and see how it runs? He probably has an H (3.8oz) or H2 buffer 4.6oz. Also make sure the gaps on the gas rings in the bolt aren't lined up.

 

[cityrice]

Cool Diagram

Where is it ejecting the spent cases? 3oz is a lightweight buffer so it may be cycling faster than the magazine spring can feed the next round in.
Did you try swapping your brother's buffer into yours and see how it runs? He probably has an H (3.8oz) or H2 buffer 4.6oz. Also make sure the gaps on the gas rings in the bolt aren't lined up.

 

[22many]

I may be way off here, but have you tried using a different mag?

 

[eldbillbo]

well seeing that you swopped the bolt carrier group out with your brother and it worked it appears you narrowed it down to the bolt carrier group .

Your using a FA carrier which means its heavier but thats not the cause unless as you mentioned the spring is heavier spring if you using a rifle lenth spring or after market stiff spring i forgot the brand but there is a after market spring that if used with the wrong set up will cause these problems so the combo could be between that heavier carrier and stiffer spring rid one and your problem could be solved . but typically these problems are caused by a loose gas key so start there is it factory staked if not tighten them as tight as you can and staked them
another thing could be worn gas rings or even doubtful but possible misaligned gas rings

I suggest you start there also make sure the bolt is lubed

and as 22many try other brand mags

you don't need to remove the gas tube to clean it and they rarely ever need to be clean as its pretty much self cleaning however its possible if you were shooting .22s or blanks it got a carbon buildup lodged in it

how is the front sight base attached ? is it a after marked bolt on or pin type or factory taper pins if its the bolt on type its easy to loosen and reposition it . if its a factory taper pin set up it may require a hammer or removal and a new gas block installed or bigger gas port drilled

 

[djgaloot]

Okay, Lots of replies and lots to check out. Just to answer some of the questions that came up.... The last configuration I was running was with my brothers Semi Auto BCG. I do not remember if it was his Stag or his spare but I was running it with his Stag buffer/spring combo and it would not cycle consistently with the Federal 55 gr ammo. From the chart above the brass was ejecting probably in the 4:30 vicinity and only a few feet away while in the standing position. The Gas Block is a Noveske that can be turned in the Normal/Off or Suppressed position. (And yes it was in the normal, correct position though I did try it in all positions). It appears to be a clamp on style though I need to check further. I do not have the rifle in front of me now. I tried several kinds of mags and all worked the same. The rifle that worked flawlessly was my brothers Stag which worked equally well with his original components and with my BCG and buffer/spring combo. This may point to a gas pressure issue or component balance issue. I need to check out all of your suggestions. Thanks.

 

[eldbillbo]

well after reading your reply it sounds like its in your gas block either not installed correctly , is slightly off center or faulty .
if its a clamp on type you may be able to twist it a little to line up the hole in the barrel and in the gas block better if its off a little it has restricted gas flow

if its a pin on type it will need to be serviced by someone know knows how to reset them .

if its a set screw type it may or may not require some expertise to re set it

If you belong to a range i would be willing to take a look at it there in exchange for some range time.

also you may want to contact Noveske and ask them what they think it might be.

 

[madcratebuilder]

Your not the only person with a 18" mid-length that well not function with 55gr Fed ammo. Solution, use better ammo. You should make sure you have a normal carbine buffer and spring before you go messing with the gas block. How new is this rifle, get 4-600 rounds down the pipe and then try the Fed 55gr.

 

[djgaloot]

Where is it ejecting the spent cases? 3oz is a lightweight buffer so it may be cycling faster than the magazine spring can feed the next round in.
Did you try swapping your brother's buffer into yours and see how it runs? He probably has an H (3.8oz) or H2 buffer 4.6oz.

Hmmm. I was always just assuming it was not cycling far enough back. I did not think about it cycling TOO fast. I did try my brothers action spring and buffer and it seemed to work better but did not cure the problem. I will have him weigh his buffer and measure the force/deflection curve of his spring compared to mine. I have a collapsible stock so should I use the carbine spring even though it is a rifle length gas system? Looking at vendor's sites is a bit frustrating as there is a distinct lack of information of a technical nature (such as the actual weight of buffers or lengths and stiffness of springs).

 

[MilesTeg]

I don't believe it is an issue of too fast. Especially since it runs fine with heavier loads (which equals more pressure being forced into the DI system). The lighter loads are causing the short stroke most likely because of a pressure issue with the gas block or gas tube. Perhaps the Noveske block is stuck on suppressed mode even though it seems you can still select the 2 settings. My bet is if you swap uppers and leave the springs and buffers alone - you will find your upper will not work well on his lower either. I just don't think this as being an issue of too light of a buffer and/or spring. If it is a spring/buffer issue then it would be too heavy as opposed to too light.

 

[EagleEyeEOD]

Be sure and look at the portion of the gas tube that aligns with the bolt carrier key. It should have a bell shape to it, scrutinize it and see that isn't worn unevenly and that it still has a bulbous end. A slightly misaligned gas tube will function the weapon fine until enough of the gas tube or the inside of the carrier key is worn away that it allows blowby, short cycling the bolt. Also check that your gas check rings are in spec. If you need to know how, PM me. Someone years ago started a rumor that you can't have all three of the gas rings lined up or it will allow blowby to the point the bolt won't function. This is absolute crap and has been taken as law in many circles. Hope that helps.