Home Forums Hunting & Shooting AR and Tactical Rifles Short cycling AR-15

Ok, so I have put together this rather ordinary 20" rifle AR-15. Nothing special, nothing weird. It short cycles BAD. Bolt slips over the magazine without picking up the next round, and just goes "click", weak ejection, just weak, weak, weak. I have only tried two different ammos so far (PMC bronze, and now some Armscor USA), and they both perform exactly the same. I assumed it was "undergassed". So....


At first, I noticed some gas blast on the front end of the gas tube, and figured it was leaking out there (after looking at other rifles that function perfectly, they also have the same bit of gas blowby marking). Well, the hole in the front sight block is within spec (supposed to be right at .180", which it is). I changed the gas tube. No effect. Still short cycling. Ok, well, maybe the buffer or spring ain't right, so I changed them with another Rifle buffer and spring I had. No effect. Still short cycling. Ok, maybe something is wrong with the bolt.. excuse me... "bolt carrier group". It's a brand new Expo Arms Microbest bolt. Quickest way to check that is to simply change the bolt... excuse me ... "bolt carrier group". So I put in one from a PSA rifle that works perfectly fine. No effect. Still short cycling. Well, schidt. What's left? The gas port in the barrel.

The spec gas port size for a normal 20" rifle with a .750" gas block journal is .093" to a max of .096". So, I take the front sight off and using drill bits as gauges, I took one that actually measured .093" and found that I could probably force the bit all the way into the hole, but I wasn't going to do that. So the hole is the proper spec size (as the manufacturer said it would be drilled at .093"). Well... ok.

I put the gun back together. At first, I was shooting with PMC ammo, which is not super high powered anyway, though I have NEVER had any problem with it before. I wondered if maybe it was a bad batch of ammo, so changed to different ammo. No effect. Same thing. The gun should not be that damn picky about ammo anyway.

Am I missing something? Basically EVERYTHING has been changed except the barrel, and the barrel seems perfectly fine, and it still does the same thing.

Gas block alignment?

Appears to be fine. There is a mark on top of the barrel around the touch hole where you can see where the gas block hole was. Now, the touch hole is right at the edge of the mark, but still fully within it, so it SEEMS ok, at least from what I can see.

Maybe the upper receiver is out of spec or has a flaw that is dragging on the bcg.

The problem HAS to be there somewhere. I will take the sight block off again tomorrow and maybe I can tell better exactly what is going on there. My eyesight is done for the day today, and I wouldn't be able to see squat right now. (edit: I went ahead and took the block off and it MIGHT be covering the hole in the barrel slightly. Still pretty hard for me to tell.)

If the gas block hole isn't fully enclosing the gas hole in the barrel (sorry, as a flintlock gunsmith, a hole in the side of a barrel is a "touch hole"), can the hole in the gas block simply be opened up a little bit to clearance it? I guess I might as well try, if that is the case.

I think the upper receiver is fine. Everything feels normal, and cycling by hand is perfectly normal and fine. It doesn't feel like it's dragging on anything. It just feels weak and sluggish and underpowered.

Wolff makes a rifle length reduced power spring, if you want to give that a shot. Brownells has it in stock for 18 bucks.

Sprinco makes a dandy reduced power spring but I think it only comes in carbine length.

bolt carrier key possibly loose and needs to be restaked or replaced?

Thank you. I was looking for reduced springs or lighter buffers, but the pickins seem mighty slim for the rifle size.

Ok, what I have with the sight block is the hole where it touches the barrel is drilled with a #28 bit (.140"), but only a short distance, then it steps down to about a size #31 or 30. I think I'm gonna try opening up the hole a little bit, but only go as deep as the step that is already there. That should clearance the hole in the barrel, but leave everything else as it is. I'll give that a shot tomorrow, I guess.

OR, the best thing to do would be to send it back to the manufacturer to let them sort it out!, but I hate doing that!

Gas tube obstructed? Gas key obstructed? gas block obstructed? Gas rings good?

No obstructions. All parts brand new, and I have tried two entirely different gas tubes, two different Bolt carrier groups, and two different sets of buffers and springs.

The more I look at it, I am coming to the unfortunate conclusion that the gas port in the barrel is partially blocked. And I have messaged the manufacturer and I'll see if they'll take care of it for me.

I really wanted this gun to be done though!

I've heard of barrel gas ports sometimes having some metal hanging on to the bore side of the port, from the drilling, and partially blocking the port.

If that's the case, I would imagine it is would be visible if you pull the gas block or FSB, and have a look into the gas port, and down the bore.

First thing that came to mind was gas block misalignment. Are you using a typical A2 Front Sight Base? Pinned or screwed? ...or are you using a clamp on gas block? Either way I'd check alignment gas block/front sight to port with a bore scope...or lacking that, with the spaghetti noodle trick. LMK if you need a description of the noodle trick.

Here is where a $50 Teslong would be your friend.

kwg

If the barrel is Salt Bath Nitride the chamber will need polished to remove the fuzzy surface finish.

A rough chamber will also induce short stroke.

The buffer spring is really a feed spring and reducing power of the feed spring is never a good plan. The proper amount of spring to feed the gun properly never changes.

What magazines?

Chrome bore. Plain M-16 front sight. A couple different Magpul magazines, which work fine otherwise. Chamber is smooth and my empties come out slick and shiny. I was never crazy about the idea of using a lighter spring or buffer, since it SHOULD work fine as-is, and changing springs is not addressing the real problem, but I would maybe try that if necessary.

It has taken a lot of strong light and two pairs of glasses, but I am now pretty well convinced that the holes are simply out of whack. At first, I thought it was OK, but now I'm fairly certain it's not. Judging by the "tattoo" mark on the barrel (which is pretty hard for me to see, but I can see it), the sight block is partially covering that gas port on the barrel. Grrrr.

My best guess is that you are correct about the Gas Port Hole not being aligned. However, it could be something else.

Did you build the upper, or did it come assembled by others? If you bought the upper assembled, send it back for replacement.

If you built it:
You are using a military style front sight correct? How is it attached? Set Screws or Cross Pins?
If Set Screws, is the Barrel Dimpled on the bottom side for the set screw to index in? Opposite the Gas Hole in the barrel.
What handguard are you using, Mil Spec Plastic Handguards or a metal free floating handguard? With a Tower Front Sight assembly the gas port hole should be aligned for a Handguard Retainer between the sight, and the barrel shoulder. This could be a problem two different ways.

Today most barrels are set up for a low profile Gas Fixture, and a Free Float Handguard. If using a Tower Sight (With Set Screws) and Military style handguards with a Handguard Retainer between the Barrel Shoulder and the Tower Sight you could be obstructing the port alignment the thickness of the Handguard Retainer.

This could be a problem the other way around also. If using a barrel made to take a Tower Sight with Handguard Retainer plate, and you are using a Free Floating Handguard again you could have a problem with gas port alignment.

If your barrel is Dimpled on the bottom for the Front Set Screw on the Gas Fixture/Front Sight. Simply align the front set screw in the Dimple.

I had one years ago that was making me pull what hair I have out. It would chamber and fire, but would not pick up another round. (Short Stroking) Took it to the gunsmith. We checked everything on the gas system. The port hole was a hair small, and had a burr. Drilled it, but did not fix the problem. The front to back hole alignment was off about the thickness of a handguard retainer. We cut the shoulder back a hair to align the gas fixture. Still not working. Finally we checked with a Go No-Go Gauge and it was chambered tight. We ran a chamber reamer in it by hand and started cutting metal out of the throat right off the bat. Apparently the manufacturer of this hi dollar target barrel finish cut the throat in a separate step, and forgot to do it on this barrel. That was the fix on this barrel.

Had another problem lately on a 204 Ruger. I did a rebuild on it. It was hard on Cam Pins originally. I had forgotten what we did to solve this problem years ago. The solution should be to cut back the gas. This barrel was .936 as the gas fixture, no adjustable gas fixture that size. I installed a Rubber City Adjustable Gas Key on the carrier. I had to have it full open to get it to cycle. I had forgotten that my brother had installed a Hydraulic Buffer way back when.

Lots of things can cause a problem.

Bob R

Can you lock the bolt open on an empty mag?

I ask because just yesterday a man came here with the same problem. He had a rifle buffer in a carbine tube so the bolt simply would not go back far enough. I had some extra buffers so I gave him a carbine buffer and the gun worked perfectly.

Another note:
Some factory barrels come with .055" gas ports. In my experience that's not big enough. On a rifle length system a .062" is a good place to start. How big is the gas port? Wire Gauge Drill size is a good way to check.
Lastly it is always worth the time on a custom built to funnel the top of the gas port. Take a1/8" drill and make a funnel at the top of the port so any slight misalignment is not critical. On GI spec carry handles and GI front sight towers I like to set the windage center on the rear and then do a basic zero by pivoting the front sight town until the rifle shoots near center of paper at 200 yards before I pin or screw the tower in place. That little tip gives you 100% of your windage adjustment in both the left and right directions. Funneling the gas port is very helpful for such rifles being made for NRA or KD paper shooting so that you never pinch off some of the gas flow by getting the front sight in exact alignment for full windage to work on the rear sight.

I've had one barrel that did that, and it turned out to be gas port size. I have long since forgotten the maker of the barrel. The local shop that sold it to me replaced it with another and the rifle ran very well afterwards with no other changes.

A great tool for gas block installation is one of the inexpensive borescopes now available, but they will open up another rabbit hole. Cleaning solvents have become my new obsession.

I assume the front sight/gas block installs with taper pin so can you see any cant?

You could use a clamp type gas block to center the gas block hole over the barrel gas port.

Might be cheaper in the long run to scrap the barrel if the port is off center. But if you think you might scrap it then you don't have anything to lose by opening the port up to compensate for it not lining up with your sight/gas block.