AR short cycle-out of options


May 8, 2004, 10:10 PM
It hasn't been shot that much, its a pre-ban upper (the lower not the problem) and I've been told by a friend of a friend that tried to get it working for me that he changed out every part he could with his working AR, it still wouldn't work. It's also of unknown manufacture. The last option we can come up with is the gas port in the barrel is to small. Some links to specs would be nice, and if any AR gurus can come up with some ideas as to the possible problem would be appreciated. I would prefer to fix this upper than to sell it and buy a new one. Fire away with any question's, I'll try to answer as soon and as well as I can.

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May 8, 2004, 11:01 PM
ammo used???

Harry Tuttle
May 8, 2004, 11:21 PM
barrel length?
buffer and stock type?
staked carrier gas key?

did anyone jam a pipe cleaner in the gas tube and block it?

May 9, 2004, 12:17 AM
ammo- different kinds, don't know what the friend used, I used reloads bought at the gun show that work well in other guns of mine.

20 inch barrel, standard fixed stock, don't know the differences between the buffer types- please explain or tell me where I can choose between pictures

dont know what the gas key is- if its the piece on the bolt carrier that interacts with the gas tube, then its bolted on with I belive allen screws.

gas tube was clear but a little dirty, the friend cleaned it (I didn't have the pipe cleaner brushes at the time) and that didn't help.

Hope that helps.

May 9, 2004, 12:41 AM
I vote for gas port too small or not in right location causing restriction. If weapon cycles normally by hand, then definately the gas port. Sorry, can't help with size. Try asking at ARFcom or roderus custom. Roderus Custom (

Zak Smith
May 9, 2004, 01:26 AM
What is the origin of this upper?

To ensure that it is indeed short-stroking, load one round in the magazine and fire it. If the bolt does not lock back, you can correctly conclude that it is short-stroking.

First, try Federal XM193 ammunition. It is mil-spec.

Make sure the bolt unlocks smoothly and the bolt carrier moves its full proper travel when pulling with the charging handle. If it hangs up, look for the obstruction. If the upper was built by someone who knew what they were doing, go back to the original BOLT - you said your friend swapped some parts around. It should have been headspaced when it was built.


May 9, 2004, 03:01 AM
Origin unknown- its unmarked, I can't find anything anywhere on the upper(the lower is rock river arms)

It hand cycles just fine, the parts that were switched went back home to their original AR , they were there just for testing to see if we (he) could find a bad part.

Bartholomew Roberts
May 9, 2004, 12:50 PM
You say the AR is short-stroking. Have you tried the test Zak mentioned to confirm that? Is it doing consistently or only intermittently?

Is the chamber of the barrel chrome-lined? Are there any markings on the barrel (.223 or 5.56 stamped somewhere?).

First, run a pipe cleaner through the gas key (the part of the bolt carrier that mates up to the gas tube) until it comes out the bolt carrier. I've seen spent primers and other debris actually get forced into the gas key and impinge on the flow of gas before. Its a freak malfunction; but it can happen.

Second, make sure the gas key is screwed down tight. On a well-maufactured AR, the screws that hold the gas key to the bolt carrier should be staked on.

One you have those two things out of the way, try shooting some Winchester Value Pack 55gr, PMC 55gr, or American Eagle 55gr. Is the problem still there? Try it also with Winchester Q3131A or Federal XM193.
Make a note of whether it happens with just certain brands of ammo.

Check the bolt carrier for shiny spots to show where it might be encountering an obstruction, though if it hand-cycles finme, this is probably not the issue.

Keep in mind before you start drilling on the gas port that a too large port can cause many of the same symptoms as a port that is too small.

May 9, 2004, 05:48 PM
It is effectivily a single shot. Short strokes every time.

No markings anywhere, barrel, receiver, sight post, handle, and a partidge in a pear tree.

If being really shiny when its clean means its chrome lined, then yes.(my k-31 is really shiny when its clean, and I didn't think it was chrome lined, so I didn't think the AR was either)

One of the parts that was swaped in the tests was the bolt carrier, but I'll check the gas key for debris and stakes(pins?)

Reading your post made me remember that I had used 2 kinds of ammo-the other one is Black Hills 68 gr match hp-but I've got some other good reload I bought at the gun show to try out, I'll try to make it to a store and get some "clean" stuff (will UMC be clean 'nough? I think some of that is here somewhere)

I'll mention the shiny spots to my dad (hes got the gun right now)
Besides the "rails" it rides on, what other shiny surfaces are 'sposed to be there?

Any permanent mods will be done by a reputable local gunsmith, so that'll lessen my worries about making new problems.

May 9, 2004, 05:56 PM
What is the spec for the gas port size in the barrel? Does the sight post match its size or does it change for engineering purposes? I called my dad and hes gonna measure and check on some questions out on the rifle this evening, and he'd like to know the spec's.

4v50 Gary
May 9, 2004, 07:27 PM
Has it ever worked efficiently with other ammo?

BTW, it would be a good "California" legal gun. What a sick thought but true.

May 9, 2004, 07:31 PM
This has been the problem from the get go. It has never worked.

Zak Smith
May 9, 2004, 08:07 PM
The Black Hills 68-77gr I have shot has been LESS powerful than the Federal XM193. You might track down a box or two and try that stuff.

Can you confirm the gas port in the front sight tower is correctly aligned over the hole in the barrel? Can you see a lot of gas/powder fouling anywhere along the gas tube's path, and ends?


May 9, 2004, 08:46 PM
Thanks for the info on the ammo, I'll take that into account.

I've allready called my dad, he's gonna check that out sometime this evening and give me a call if he finds anything amiss.

It might be misaligned, I tend to take everything apart just to look at it when I get a gun. And yes, I did put it all back together by my self. I just might of not put the sight post back on right. (he knows this and is gonna look, and yes, I know I didn't need to take the sight post off, I just couldn't stop myself.)

May 9, 2004, 08:54 PM
Ok, called my dad again, he said the friend used Winchester and other mil-spec ammo, plus some of the black hills that was in the mag I sent with the gun.

He(my dad) wants to know what the gas port on the barrel is supposed to be, hes gonna measure it when he gets home. Does it depend on manufacturer, or is it universal for an AR-15 20 inch barrel ?

Bartholomew Roberts
May 9, 2004, 08:54 PM
zpo, is it trying to extract the spent case at all? If not, it is unquestionably a gas system issue and indicates a total blockage somewhere. If you swapped bolt carriers and the problem persisted then it is in the gas tube or the gas port (misaligned, too small or non-existent but not too big).

If it is trying to extract and eject the case (stovepiping/double feeds) then it may be any number of issues.

May 9, 2004, 09:00 PM
It will fire and eject the round in the chamber, but will not pick up the next round.
It doesn't stovepipe, the bolt doesn't go all the way back to engage the next round.

May 10, 2004, 01:49 AM
Well, I didn't get the evening checkup obviously, so I'll post when I get it.

May 10, 2004, 05:34 AM
Here's an exelent tutorial on short stroking.

Short-Stroking FAQ

Courtesy of Rick McDowell

Competition Specialties, NRA Life, USPSA Life, Master

( 105 E. Cass, PO Box 451, Osceola, Iowa 50213 Phone: (800)369-4481 / (641)342-2011 )

Q: What causes short-stroking?

It is not necessarily high pressure that causes short stroking but higher pressure at the gas port. This can be a delayed or longer pressure curve caused by a slow burning powder. A faster burning rate powder will have a higher pressure spike earlier in the burn and then diminish as the bullet goes down the barrel. When using a slower burning rate powder, the duration of the burn is longer (in time), so when the bullet gets down to the gas port the pressure is actually higher than it was with a fast burning powder.

In an AR15 type rifle (and others that use a rotating bolt design), there needs to be a way to unlock the bolt immediately after firing. A small amount of gas is used to do this job and the gas is taken via the gas port in the barrel. I am sure an incredible amount of experimenting went into finding just the right location as to where to put it and the size of the hole. The gas is directed down the gas tube and into a chamber in the carrier directly behind the bolt. When this chamber is filled with gas, pressure is equalized and the only surface that can move, is the back of the carrier chamber. When it moves the carrier back, it rotates the bolt cam pin and that unlocks the bolt. There is not enough pressure available to drive the carrier assembly all the way to the rear so the unlocking has to be done quickly after firing so that there is still some residual momentum from the cartridge driving back against the bolt. When the cartridge fires, the brass case is expanded under tens of thousands of pounds per square inch of pressure and it will then grip the chamber wall very tightly. In fact, I you have an over headspaced chamber, the firing pin will drive the cartridge forward in the chamber at the moment of firing and it will stay forward because of the tight grip on the chamber walls. That leaves a small amount of space between the head of the case and the bolt face. As the pressure subsides the case will grow a little longer and contribute to the brass flow and thinning of the case walls. This is partly what causes case head separation. This happens a lot more in the M1 or M14 rifles.

Now comes the cause of short stroking. In a rifle that may have had the gas port drilled out (or just drilled wrong from the factory), and/or a cartridge with a real slow burning powder, the gas that is received into the gas system is much higher than is needed. This high pressure gas fills the gas chamber much faster, causing it to try and unlock the bolt way too soon. At this point in time the extractor in the bolt is pulling on the rim of the cartridge very hard but can't pull it out of the chamber because the case's grip on the chamber walls hasn't yet subsided. The timing of the whole gas system has been speeded up and now it is wanting to try and extract a case that is to tight to extract. When this happens, some of the momentum of the bolt and carrier is lost because the case didn't easily come out of the chamber. A tell tale sign of this is a deep extractor mark on the inside of the case rim. In a properly functioning rifle, the extractor doesn't actually extract, the case is actually driving the bolt rearward. The extractor just holds the case in the bolt until the ejector gives it a flip. Now that the carrier assembly doesn't have enough inertia to travel all the way to the rear, because the grip of the case in the chamber slowed it down, bingo, you get a short stroke that won't go back far enough to lock open or cycle another round. Many times the bolt will come back just far enough to hook the next cartridge in it's extractor groove and try to feed it into the chamber this way. You will get a big dent in the side of the cartridge along with the failure to feed. Some people who don't understand this timing will then think that the ammo must be under powered or that the carrier is not getting enough gas to it. They then drill out the gas port hole believing that this will get more gas to the carrier to force it rearward harder. It does, but now the bolt is unlocking even faster when the case is under even greater pressure. When this doesn't cure the cycling problems, they are then really confused and will sometimes drill out the gas port again!

Another thing that will contribute to the short stroking is a rough chamber. I have seen these especially with the cheaper bottom feeder type barrels. These barrels are chambered with reamers that have seen way too many barrels and needed to have been replaced a long time ago. A mirror finish is not actually necessary in a chamber but it should be smooth and not have any grooves in it from a chipped chamber reamer. I have cured many of these barrels by chucking the barrel in my lathe and using a series of SE tool makers stones to stone the chamber walls smooth. On a really rough chamber I might start with a 320 or 400 grit stone, then a 500, then a 600, and 800, a 900 and finally a 1000 grit stone. These are stones and NOT sandpaper. Stones will remain rigid and not follow the hills and valleys of a rough or uneven chamber surface. These stones will also 'dissolve' to fit the shape of the surface I am stoning on. In just a few strokes the stone will match the same radius of the chamber. By the time I work down the to the 1000 grit stone the surface will be very smooth and more importantly, free from and grooves or deep scratches that can cause rough extraction. This stoning only removes a few ten thousandths so there is not problem with making the chamber oversize.

Another reason that some rifles will cycle better than others is that not all chambers are cut to the same width. Military style chambers tend to be a little wider by a few thousandths so that the grip of the case on the chamber walls will break loose a little sooner than a rifle that has a narrower 'match' chamber. Companies that like to use 'tight' chambers to make people think that their rifles will give them better accuracy, will have rifles that are also a little more prone to notice differences in ammo pressure levels. Tight is not always better for accuracy. A cartridge and chamber that match each other perfectly with just the right amount of play will be more accurate than a chamber that is tight on the cartridge.

So if you are experiencing a short stroking problem you need to know what size port you have in your barrel. You also need to try different brands of ammo and see what happens. A while back it was the Malaysian surplus ammo that was causing this kind of problem. Lately (1999) the Winchester white box has been giving people fits (Note: ammo is fine now in 2001). Maybe it is only the ammo. Maybe it is the person's rifle that is the real problem but they are blaming it on the ammo. The only way to know is to start eliminating the variables. Overall the M16/AR15 is a very reliable rifle and shouldn't be finicky about different ammo, within reason of course. I sell the one piece gas rings and once in a great while I hear from a customer that said his rifle will short stroke after putting in the new ring. Upon examining the rifle I find out that the rifle had gas system problems but with the original leaky rings, the problem didn't surface. When the new ring was installed the system became much more efficient and the high pressure problem came into light. I have been able to save several barrels with too large of gas ports by installing a bushing in the barrel. I mill a larger hole into the barrel and fit in a tool steel bushing. I am then free to drill my own favorite size of gas port and dial it in to the brand(s) of ammo the customer is using.

Q: Are there other causes of Short Stroking? I have an Extraction Problem, is this related? How can they be fixed?

One way to tell if your AR is having an extraction problem is to look at the rim of the extracted cases. On a rifle having a problem, there will be an extractor mark deeply into the rim. It shows that the extractor was trying hard to extract the case but it wouldn't come out. The case from a normally operating rifle will show very little extractor marks. I have seen extractors almost pull the rim off of the case when trying to extract it. If you have a short stroking rifle, here are a few things to look for;

1. If it is a new factory rifle, just shoot it more and keep cleaning off the carrier parts and lubing them. Chances are high that it is just too new and it will take a while for the parts to wear in and become slicker. The coatings that are used on these guns cause a lot of friction but will wear in shortly. If you still are having trouble after 200-300 rounds, look for other causes.

2. Check your ammo. Try different ammo. Try your ammo in a friends rifle. Look for pressure signs, flatter than normal primers, firing pin dents that have been pushed back up, holes in the primer, deep extractor marks, brass that has been extruded into the extractor hole, etc. Chronographing your ammo would be a good idea, especially if they are reloads. Chronographing doesn't tell the whole story with your ammo, as using a powder that gives an early, high pressure spike may still give normal velocity. Been there, done that. ;-)

3. Check the carrier key for leaks. Look for signs that gas has been blowing out from under the key. Check the bolts for tightness. Sometimes I will make sure the bottom of the key is perfectly flat by stoning it or grinding it on a surface grinder(lightly!). On my match rifles I will use red Loctite to make a better seal, just in case.

4. Check the gas rings. A properly functioning rifle will usually work even with the ring gaps aligned. If your rifle works when they are unaligned, but doesn't when they are aligned, look for something else that is contributing to the problem. Or better yet, get one of my one piece gas rings. They have no end gaps and are much higher quality than the three piece rings (shameless plug). ;-)

5. Check for things like the gas tube being installed out of alignment in the front gas housing. It is rare but it does happen.

6. Pull the front gas housing off and measure the diameter of the gas port (best done by a gunsmith!). Also measure the diameter of the barrel also as the size of the hole varies with the barrel diameter. For one example, on a Colt 16" lightweight barrel the gas port diameter is usually .063"-.070". On a Colt 16" heavy barrel the port is opened up a little to approx. .075". The distance that the port is from the muzzle will make a difference also. On 10" barrels the port needs to be a little bigger, say .093", because the pressure drops off fast after the bullet leaves the barrel. [See the next Q&A for more info on this]

7. Check to be sure the buffer spring is correct and lubed nicely. I grease mine to keep them quiet.

8. Check to make sure the hammer is not sticking up too far and catching the firing pin on the carriers way forward. I have seen this a lot on rifles that have had a 'trigger job'. Metal was removed from the front of the trigger and from the hammer hook to get a nice trigger feel, but the hammer now sits much more rotated forward and will interfere with the firing pin. This happens the most with the hammers that have the notch on the top corner. The notch will catch on one of the rings on the back of the firing pin.

9. Check the chamber for grooves or ridges left from a damaged chamber reamer. I have seen this a lot from the bottom feeder companies and even a few of the 'better' name brands. The factory tries to get more life out of the reamer, or had someone that is rough with it and puts a nick in the cutting edges. These ridges and grooves cause more grip between the case and the chamber wall. [See the above Q&A]

10. Make sure the bolt and carrier are free to move forward and backward inside the upper receiver and that everything looks like it is aligned. Make sure that the magazine isn't hitting the carrier anywhere either.

Harry Tuttle
May 10, 2004, 11:36 AM
i was just gonna post that


May 10, 2004, 07:29 PM
On production barrels there is not a lot of care taken when drilling the gas port, so often there is a metal chip (burr) hanging down from the port into the bore. When the first shot is fired the burr can be driven back up into the port or gas block and jam up the works. Pull the gas block and make sure it is clear, the gas tube is clear , and the port itself is clear. There is a good chance that you will find a metal chip somewhere in the gas path.

Tony Rumore
Tromix Corp

May 10, 2004, 09:33 PM
Didn't mean to steal your thunder man. :D

May 26, 2004, 05:05 AM
Well, to bring this back up... Got a new front sight post, gonna see if that fixes the problem. My Dad found a sight with measurements, and everything is in spec. Need to get a new set screw, and then test it out. Thanks for all the help so far.

June 18, 2004, 06:34 AM
Okay, its all over. The new sight post fixed my problem.

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