One of the common hiccups with the AR is short stroking… often you will not even know it is doing it, until your rifle starts to fail to pick up a new round. One early indication of problems is that the bolt does not lock back on an empty magazine. Let’s look at a few things that are common cause of the problem. All of this is assuming that you are running full power, good name ammunition. If you are shooting “Uncle Festers’ Bargain Brand Reloads”, get a box good ammo and see if you still are having problems.
First, and with nearly any problem with the AR – try a new magazine! I will talk more about the US magazines and our fighting doctrine concerning them, but if you are having any problem with your gun, switching magazines is almost always the first troubleshooting step -- even if the mag you are using works fine in another gun, that does not mean that it is fine in every gun! If you identify a bad magazine, get rid of it. Period.
Some magazines set too high in the well and cause the lips to rub against the bottom of the bolt. In some cases it is enough to slow the bolt down enough to short stroke, at times it can be enough to lock the bolt back from returning to battery.
The next easy thing to look at is lubrication – check out Pat Roger’s thread on here about lubrication… do not be afraid to lubricate the rifle, it is a machine.
With those easy things out of the way, let’s troubleshoot the rifle for some other common things – working from the front to the back.
The gas block – The front sight, gas block assembly should be looked at to make sure that it has not moved and is not leaking. If you have a cross pin mounted setup, most likely it has not moved, but the set screw type blocks can move (usually forward) or twist and block off the gas port. Carefully check to see if it looks like the gas block has moved, you can usually see marks that indicate it has moved… it only takes an eighth inch to cause problems.
Also check for leaking, the most common sign is a brownish, rust looking stain around the barrel where it passes through the gas block. Some people will tell you that this is normal and will “seal” with time – it may be normal, but it is not right. The TDP specs for the barrel to gas block fit are one of the most exact measurement on the gun, this is a zero tolerance fit and is not supposed to leak at all – if you are getting leaking here, I strongly suggest talking to the supplier or builder about making it right.
The gas tube – Make sure the gas tube is tight in the gas block and again is not leaking. Double check the roll pin that locks the tube into the gas block, this is all that holds the gas tube and if it has worked out a little the ports will not line up. Check to make sure the tube is not bent or ruptured… The fit of the tube in the gas block is another very precise specification and there should be no leaking around the tube where it fits into the gas block.
Moving back to the bolt assembly, first take a look at the bolt key – it must be tight. The two screws that hold it on should be torqued and locked by staking. If you can wiggle or move the gas key any at all, the key needs to be removed, the surface under it cleaned and then properly reinstalled.
Pull the bolt and check the gas rings… these are much more forgiving than many people think, I have seen guns work fine with rings broken, missing, or the gaps all lined up -- but just to make sure, check that they are in good shape, not broken, worn or missing and stagger the gaps so that they are at 3, 6 and 9 o’clock.
Moving further back, check the buffer to make sure it is the proper weight for your rifle, running too heavy of a buffer can cause short stroking, especially in a rifle length gas/barrel system – also check the recoil spring, an over length or “heavy” spring can cause problems at times.
These are the “quick check” items for initial troubleshooting – if you run through these and still have a problem, let us know… someone on the board will get you fixed up quick, fast and in a hurry!
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Originally Posted by jmart
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