Some more info on short stroking:
Q: What causes short-stroking?
A: It is not necessarily high pressure that causes short stroking but higher pressure at the gas port. 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. 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.
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. Overall the M16/AR15 is a very reliable rifle and shouldn't be finicky about different ammo, within reason of course.
Q: Are there other causes of Short Stroking? I have an Extraction Problem, is this related? How can they be fixed?
A: 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. If you are still having trouble after 200-300 rounds, look for other causes.
2. Check your ammo. Try different ammo. Try your ammo in another 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.
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.
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.
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. Also measure the diameter of the barrel also as the size of the hole varies with the barrel diameter.
7. Check to be sure the buffer spring is correct.
8. Check to make sure the hammer is not sticking up too far and catching the firing pin on the carriers way forward.
9. Check the chamber for grooves or ridges left from a damaged chamber reamer. These ridges and grooves cause more grip between the case and the chamber wall.
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.