Has your AR Short-Stroked?

Ever heard of a pistol or a rifle “short-stroking?”  No, it has nothing to do with the 1981 hit single “The Stroke” by Billy Squire (what was that all about, anyway?)  Firearms are said to “short-stroke” when the bolt (or slide) does not travel far enough rear-ward to eject the spent case, cock the firing mechanism, and chamber a new round.  Technically, short-stroking can occur with any action (yes, even manual bolt-action rifles – think about it.) but it is most associated with semi-automatic and automatic firearms.  This short Youtube video shows a couple good examples of short-stroking.

So, why my interest in sharing about short-stroking?  I was at the range shooting my gas-impingement AR the other day.  I planned to shoot 40 rounds, in two magazines of 20 rounds per.  First magazine, no problems.  Shot it pretty well for a guy with older eyes.  Second magazine… problems.  The bolt stopped cycling at all (the no-stroke!)  I could manually operate the charging handle to eject the case and chamber a new round, turning my nice AR into a bolt action rifle.  Like most people, since the first magazine had worked and the second did not, I figured “bad mag” or “something broke.”  Let the diagnosing begin.

Ok, specifically with short-stroking, what can go wrong?  Remember, the bolt is not traveling far enough rear-ward to eject the spent case, cock the firing mechanism, and chamber a new round.  Therefore, bolt can be obstructed, something can be slowing the bolt down (too much) as it travels rear-ward, or the bolt isn’t getting enough “push” from gas or the piston.  Why this is happening can be explained by a few hundred things (that’s an exaggeration.  For those of you that know me, I like to stick to the most likely.) However, if you are interested in a lively discussion many of the possibilities, check out this thread from Calguns.

Most of the time, short-stroke problems are caused by insufficient gas to operate the piston or bolt.  Let’s assume we have a gas impingement system and we have two fully functional AR’s (because one is never enough, right?)  Not enough gas could be caused by:

  • Something wrong with the bolt/ bolt carrier (gas rings, gas key, bolt and/ or carrier ports)
  • Something wrong with the gas tube (separated from the gas block, kinked, broken)
  • Something wrong with the gas block (shifted off the port)
  • Something wrong with the gas port (plugged or too small)

As with anything on the internet, you perform any of the actions in this article at your own risk.  I specifically suggest that people who are not qualified to work on firearms bring them to a qualified armorer or gunsmith.

That being said, here’s a quick way to diagnose the issue.

  1. Swap the bolts/ carriers in the ARs.  NOTE – there is some risk here.  It is possible (but not likely with mil-spec ARs that are chambered for the same round (e.g., 5.56)) there may be a head space problem with swapping the bolts.  If you are not sure, SKIP THIS STEP.  Do they both work?  Yes?  Not the bolt/ carrier.  Does one work and not the other?  You’re right,  it’s the bolt/ carrier. Check the key, the rings, and the ports.
  2. Visually inspect the gas tube.  Is it connected to the gas block?  Is it noticeably bent, kinked, or broken?  Is the gas tube pin still securely in the gas block?

    A2-type Front Sight and Gas Block
  3. Check the gas block.  Is it installed permanently (e.g., as in some A2 gas block/ front-sight) or is it a “low profile” block installed with set screws.  If it is not permanently installed, the gas block may have shifted.  In the case of the set screws, this is probable, especially if the barrel was not dimpled and/ or the screws were not set with a thread-locker.  Look for discoloration around the base of the gas block and the barrel.  If you see it, gas is escaping, and that isn’t good.
  4. Check the gas port.  Attach a small piece of rubber tubing to the end of the gas tube in the upper.  Blow through the tube and listen to learn if any air comes out of the gas port in the barrel.  If so, that’s good.  If not, the port is misaligned with the gas block or the port has become obstructed.

My guess is that one of these checks will help you quickly find the issue.

My problem?  I started simple and checked the mags.  You already know that the vast majority of feeding and ejection problems are related to magazines.  Not enough “spring,” too much “spring,” bent feed lips, bad mojo, whatever.  No problems, though.  Both were factory Colt and both were in good shape.  Both worked Ok in my other AR.  Not the mags.  I swapped the bolts/ carriers, no issues.

Moving along to the gas system.  For this AR, I had purchased a complete upper from a manufacturer with a low-profile gas block and a free-float quad rail.  Off with the rail so I could look at the gas block.  Lots of discoloration and it seemed to have moved forward towards the muzzle.  The block was installed with set screws.  No dimples, no thread locker.  In fact, looking more closely and shifting the block a little, the gas block had shifted forward nearly choking off the gas port.  Ah, simple.  Completely remove the set screws and realign the gas block and gas port.  Use thread locker to reinstall set screws.

Time to test fire.  Load and insert a mag with two rounds.  Manually chamber.  Bang.  Good ejection.  Good chamber.  Bang.  Good ejection.  Lock back on empty mag.  Fixed!  Re-install rail.  Get a beverage.

That’s my story on my short-stroke adventure.  If you have any similar issues, this may help you get to the bottom of them.  Or, you can always give me a call.

For a more complete article on short-stroking, check out the article by Foghorn in The Truth About Guns.