What usually cause an AR to short stroke?

BuzzBee

BuzzBee

New member
To Mike D-tech and all the AR buffs,

I’m trying to help a buddy to trouble shoot his new upper, unfortunately I know just enough to be dangerous, hoping you guys can offer some guidance or maybe direct me in the right direction.

Here is the information he gave me, I’ll try to summarize them and put it in fewer words for you.

He installed a new 223 flattop upper, 20” 1-9” barrel with a new chrome bolt assembly. The lower is by Colt, prior to this, the gun cycled flawlessly with original 16” A2 upper.

He tried 3 different magazines (Colt and Borwnells), a single shot sled and even with the magazine removed, all have the same problem only extract the brass and ¾ ways but failed to eject.

He tried Wolf(sp) 55g, 60g hand load and each failed to cycle, he is a seasoned handloader, he did say they all shot very accurately.

He removed the gas block and checked for the port to block alignment also exam the gas tube and they looked kosher.

The gas tube to bolt alignment looks good and the gas tube is centered in the hole of the lug.

He also tried with his other bolt but the problem still exsist.

Any thoughts on this ?

Thanks in advance.

Buzz
 
Quote:
I would think the one he has should be OK as it worked fine with the original A2 upper, don't you think?




It's where I would start and it's what I consider the most likely culprit, considering what you've already ruled out.
That action spring has been the cause of short-stroking in a lot of ARs before, and it's a cheap fix.
 
What's the correct port dia? Any idea? I assume the port is round to mate the gas tube, am I right?
whats the proper distance from the gas port to muzzel on a 20" barrel?
 
Thanks guys, I copy and paced everything down, I'll print out the list and take with me to the range tonight.
I am also bringing my gun for him to interchange the parts and see if we can narrow down the cause.
 
The first thing I would check is weather the gas block has been bumped. If it is of after market type and not the stock type with the tapered pins it could have moved. I would need to inspect the weapon, but it could be caused by any of the issues raised above.
 
I read through the responses quickly, so forgive me if I am repeating.

When you remove a gas-block, after you have shot an upper even only a few times, the barrel will have a "shadow" of the port in the gas-block around the gas-port in the barrel. Hopefully, this "shadow" is concentric with the port in the barrel. If the gas-block is misaligned enough, it will cause short-stroking.

I see someone has mentioned the carrier-key getting loose. this will indeed cause short-stroking, and if loose enough, it will lock-up the bolt in the upper receiver. Check the two screws to make sure they are tight. The next thing I would check is to make sure the gaps in your gas-rings are not all lined up. The gaps should be staggered roughly 120 degrees from each other.

Check the fired brass carefully. Look for radial lines around the brass or other impressions in the brass. If you have a chamber that is rough or has rings from a chipped reamer, you can have extraction problems just like you are talking about. Most of the time a rough chamber will imprint the brass, not always, mostly.

It would be odd to have the wrong sized gas-port in a barrel. Most barrel assemblies are mass produced and the tooling is not changed back and forth. I have had one that was sized incorrectly, but almost unheard of. I use a #37 drill for my 20" H-BAR and a #36 for my 20" Bull barrels.

Those are just a couple of things I would try first. Let us know what you find and perhaps we can help you more with a little more information.
 
Thanks guys, I think I primed with enough information to keep us busy tonight. I'm also taking a few boxes of M855PD with, I know they work in my gun that way we can rule out the Ammo.
One more thing, I know the carrier key screws are staked (at least my are) is there any tricks to removing them? How should I go about restake them during reassembly or may I use red Locktite?
 
If the screws are tight, don't mess with them. There is no reason to take them out. Red Lock-Tite, or any other color for that matter, won't work on the carrier-key screws. That key gets like the core temperature of the sun after a few shots.
 
M855PD is not the best ammo in the world to be accessing the function of a problematic AR15. Go to AR15.com, read the ammo oricle, you'll see what I mean. Try good quaility ammo, not wolf which is very often lower pressured ammo.
 
If this guy has shot Wolfe ammo in the new upper it could be it is gummed up with the junk on the case to keep it from rusting. It is some type of junk that will coat a chamber and screw it up for function. I believe that Wolfe has changed this stuff out but he may still have some of it. I would change out the bolt-carrier in the upper with one that is known to work and see what happens. It this works you have narrowed it down to the bolt-carrier. Sounds like the ammom does not have enough pressure to cycle the action properly.
 
Wolf has been Poly Coated for a couple of years, the Poly ammo works pretty good, although Wolf in general is loaded a little light and sometimes would short stroke, it was mainly the old lacquer coated stuff that could cause problems.

Keep in mind that Wolf doesn't work in all rifles, it works OK in most, but some rifles just won't work right with Wolf ammo.
 
I thought I would let every one know about my venture last night.
As it turned out it was a faulty gas block, it was the upper half of the block, it didn’t mate the barrel perfectly. Upon removing the hand guard, I found a shadow on the barrel from the powder residue escape from the leak, then after I removed the gas block, I can clearly see the tool mark chattered right where the gas port is located.
He is sending it back to the maker have it replaced, other then that it looks like a very nice upper.

Thanks again,

Buzz
 
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