A common problem I see here in my shop is an AR style rifle with the gas port drilled out too far. The owner usually had a short stroking problem and assumed that it was getting too little gas to cycle the bolt and carrier all the way to the rear. He then drilled out the gas port in the barrel a few sizes and found out this didn't help at all but made things much worse.
With all the foreign military ammo floating around these days, pressure problems have risen. A lot of this ammo was made for loose chambered machine guns where over pressure was not a problem.
An over pressure round will cause short cycling quite often but drilling the port out is not the answer. What happens is this; upon firing the over pressure round, the case is instantly expanded under all the pressure and forms a tremendous grip on the chamber walls. An equal amount of pressure is also pushing on the bolt face and bolt lugs. When the bullet reaches the gas port hole in the barrel, there is much more pressure than is really needed.
The pressure is routed down the gas tube and into the gas chamber in the carrier, just behind the gas ring(s). This gas fills the chamber, expands, and causes the carrier to be forced rearward.
When the carrier moves rearward, it causes the bolt cam pin to rotate the bolt so the bolt can clear the barrel extension lugs. Since the excess gas pressure makes all of this happen MUCH more quickly than normal, the bolt and extractor are trying to pull the fired case out of the chamber much SOONER than designed. The fired case is still under a lot of pressure and has not released its grip of the chamber wall yet.
There is no way the extractor can pull it out until the pressure subsides and the grip is released. What happens is that the bolt and carrier have tried to extract the case but the grip was too much to overcome. In an instant after this attempt, the case comes free from the chamber so it can be extracted, but the momentum that the carrier had, has been somewhat used up trying to extract an un-extractable case. It doesn't have the inertia anymore to cycle all the way back and will give you a short stroke and probably a jam.
If you switch over to a normal pressure load, the rifle should work fine (barring any other problems). With normal ammo, the grip of the case in the chamber will release before the extractor attempts to pull it out.
Actually in most cases, (for semi auto rifles, pistols, and shotguns) the extractor is NOT pulling the cases out of the chamber. The case is actually driving the breech rearward. Take a look at the little extractors on most semi auto shotguns. It wouldn't hold up to having to pull shell out of a chamber very long.
Military machine gun chambers are on the loose side of the spectrum and when they fire a hot cartridge, it will not grip the chamber for as long a time because of how far it has had to stretch. It will release sooner and not affect the timing of the extraction. Standard SAAMI chamber specs that come on most AR rifles are just fine for the most part. Tight chambers don't mean better accuracy. I like my match chambers to fit my sizing dies, whether the chamber is bigger than SAAMI or smaller means nothing. It is how the brass fits the chamber you have that really matters.
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 friendâ€™s 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. 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.
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 carrierâ€™s 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. I have hand stoned out many chambers with SE stones to make a mirror like finish.
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.
Hope this helps with curing a short stroking problem you may be having. I learned a lot of this from making pistol compensators. The longer and more efficient I made them, the more I had to have the timing correct. Rifles are no different.
Rick McDowell NRA Life
Competition Specialties USPSA Life,
105 E. Cass, PO Box 451 515-342-2011
Osceola, Iowa 50213 800-369-4481