Here's an exelent tutorial on short stroking.
http://groups.msn.com/TheMarylandAR15ShootersSite/shortstrokingfaq.msnw
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.