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.

which size drill bits do you use?

I believe he said a .093. That sounds like the perfect size of port. I'm betting it has to do with the port location or the gas block location.

kwg

I have reamers in stepped sizes but a drill works if you feed it slow to reduce the burr in the bore.

The mil spec for rifle gas on 20 inches is .093 but thats a bit small if you want PMC bronze to run when a bit dirty or cold.

Rifle gas is really forgiving and I would run a .100 at a starting place if OP can't get the block and port to line up.

I like guns that run so if it's going to run dirty and cold it's going to be a bit overgassed when clean and warm.

What Burns said.

All other things being eliminated as an obvious problem, the OP doesn't have enough gas.

I've even had a problem with PMC Bronze with a 16" KAC being undergassed in cold weather.

Changing to full power 5.56 NATO spec ammo will work under same conditions.

Ream the gas port to .103" & stop wasting time.

MM

I have an 18" barrel with the .093 and it's gassed perfectly. I would be reluctant to open up the port but a .093 would be perfect for making sure any burrs are gone.

kwg

.093' on 18" barrels usually works fine................but may be marginal with some low powered ammo & cold weather. I've seen it more than once.

It's easier to deal with a little extra gas than to deal with a shortage of gas, either via an AGB or different buffers & springs.

JMHO

MM

It's a rifle length gas system with 2 inches less barrel. I used the drill bit shank to measure the hole. I guess I could of mike'd the shank to confirm it's dimensions.

kwg

Crane mil spec is .105 for the 18 inch rifle gas and that's for the MK12 issued with a suppressor and shooting mil spec 5.56.

Unsuppressed a .105 port/18"/RG will choke in my world on PMC bronze unless very clean and it's warm. Suppressed is fine.

I have run .110port/18"/RG a lot and it works fine both suppressed and unsuppressed.

I think altitude may also have an impact on gassing.

Good stuff John.

Thanks.

Do you have a source for that?

MM

Yes.

Well, I'm too impatient, so, I went ahead and made up a jig so I could hold the front sight securely on the drill press, and worked the hole over in the direction it needed to go so that it easily catches the entirety of the gas port in the barrel. It's oversize now, but only just partway in, there's still the step down in the hole going towards the gas tube, and since it's just a passageway, I figured it can't hurt. Now, after firing a few shots, I have a gas mark on top of the barrel that is fairly concentric around the gas port hole in the barrel. All should be clear now.

But.... still weak. I only fired a few shots, and though they all cycled and functioned, ejection was weak, and the bolt didn't hold open when empty. Granted, this is not super powerful ammo, but it really ought to work with whatever I put in it. The gun shouldn't be that picky. Tomorrow when I have some daylight, I'll run some actual 5.56mm ammo through it and see what difference there is, if any. I expect that I will need to open up the gas port in the barrel, which I can do. It is currently a tight .093". Now, this has a .750" gas block journal diameter. Some of the charts show that for the original 5/8" barrels, the standard gas port would be .093", and for the 3/4" barrels, should be .096". I'll probably end up giving that a try tomorrow, after shooting it some more. We'll see.

BBQ skewers fits down the barrel perfectly to protect the offside while drilling.

Well, post it up, no need to keep it a secret...............I assume they have the correct specs for other length barrels & gas systems.

All I've ever really been able to find is various non-certified info, some of which seems right, some is questionable.

MM

You're not going to make it at .096". Man up & take it to .103 -.105".

MM

I did post it up. Crane spec is .105 for the MK12/SPR, has been for over a decade.

You're welcome to check on Snipers Hide.

You do realize the OPs barrel has 2 more inches of dwell than the MK12, right?

20" rifle gas runs a smaller port than 18" rifle gas.

Yep, sure 'nuff, John. I always attempt to verify info that I can't readily find a source for. But you are on your toes...................LOL.

And yes, I do realize that 20" may be smaller than 18" RG because of the added dwell time. But I also realize that more of the powder has burned in the longer barrel, which lowers pressure, requiring a bigger gas port. More that just one variable.

But, nonetheless, assuming the OP has no other issues that he hasn't found or told us about, he is simply short of gas with that ammo, at whatever temp he's shooting with a .093" port.

And also, as I said earlier, it's easier to deal with a little over-gassing.............compared to being under-gassed, especially if he wants to shoot lower powered ammo.

YMMV

MM

This discussion about increasing the flow to the gas system is interesting to follow along. I come from the other side of the coin. In the Service Rifle competition world, we tend to shoot loads that are on the warmer end of the spectrum (if you consider a 77 SMK at 2800 out of a 20" on the warm end). And we're trying to tone down the gas system. Mostly it's to preserve brass life and extend the life of our moving parts as well making recovery of our brass easier. The cam pin tends to take a beating as does the channel in the upper where the cam pin bears on our rifles.

We employ things like reduced size gas ports, heavier (H2 and H3) buffers or in a previous generation the Tubb CWS. I've always worried about increasing the mass of the reciprocating assembly beyond what the system was designed around. So what I'm left with is reduced gas port or more sane loads.

Considering that the OP is looking for the opposite, I'd suggest the opposite of our solutions; loads closer to milspec first then troubleshooting why it's not functioning properly. Lightening the recip assembly will probably cost you a chunk in parts and opening the gas port would be irreversible and narrow your ammo choices, so they'd be my last choices.

The AR is quite amazing when you consider the wide range of conditions and loads it's expected to operate in. However being an easy to DIY type rifle can make it prone to errors in parts fitment and assembly.

There is no “one way” when dealing with the general public. The MIL got as close as you can get by using a set pressured ammo and making sure it runs in the +120 to -20 degrees.

With folks like me burning as much powder as I can then you have folks using the cheapest (weakest) stuff they can find, it’s hard to set up a gun to run both equally well.

Over gassing one just to appease the guys running cheap ammo won’t work with guys that use good ammo and expect a smoother shooting gun. Can or not.

All true from the last 2 posters, but AGB or adjustable gas keys are made for fixing any over gassed issue allowing a wider range of ammo, assuming the weakest ammo will function at the size of the gas port & a full open AGB

Cold weather will wreak havoc on marginally gassed systems & weak ammo, & I mean temps in the 20's & 30's.

MM

We all have our standards.

Ok, my gas port is now drilled out to be .096", which, best I can tell, is actually the proper diameter for it to be with a .750" journal.

Actual M193 functions fine so far. Cycles, locks open, the whole nine yards. I've tried the PMC Bronze, and also some Armscor USA, and also some steel case Wolf military classic. All these cheapies will function about half the time, otherwise, the bolt slips over the cartridges or a cartridge starts to nose up and the bolt jams into the side of it. None of them lock back on empty. This is not a problem I have ever had with a carbine/midlength before. I guess that the gas pressure is just so much higher, it'll shuck through anything.

I will probably drill it out larger, but not today. I would like for it to be able to digest whatever ammo I put in it. But at least it is going with good ammo. I wonder if the dreaded "overgassing" is such a big problem with a rifle as it is supposed to be with a carbine, given how much softer the rifle action is to begin with.

Hell, I'm used to a Mini-14, which will slam its way through anything, and chuck the empties fifteen feet away!

It's a nice day today, at like 65 degrees. I don't want it to choke when it's 30.

.093 is the mil spec port for 20"/rifle gas and runs stuff like PMC bronze OK at warm temps.

Rifle gas works fine but you have told us that the gas block is not aligned with the gas port. Middy gas or carbine gas will have the same problems with misaligned block/port.

With a misaligned block/port when you drill the port oversize you are attempting to create the same gas flow of a .093 port in an aligned setup. I would not be afraid of a .096 port in an aligned 20"/RG because the .093 20"/RG is marginal for low powered ammo and really dirty/cold.

To much fuss has been made of overgassed for most situations especially in a forgiving setup like the 20"/RG. There was a time where some companies were sending out Carbine gassed guns that were way overgassed but the pendulum has swung back a bit and now it's more likely to get a AR undergassed for cold or dirty conditions than one overgassed.

My gas block is now clearanced so that the gas port in the barrel is fully within the opening. The port is now .096". It functions fine so far with m193, but still short cycles on the cheap stuff.

Before I go drilling out the gas port hole any more, I'm gonna try something. You can get a spacer (kind of overpriced, but whadda ya gonna do?) from Spike's Tactical whereby you can put this spacer into the rifle buffer tube, and then use a carbine spring and buffer. This way, I can try different buffer weights, since there are a plethora of different carbine buffer weights, and pretty much only one rifle buffer weight. I'll see where that gets me.

Good lord…

I don't believe that the way you clearanced the gas block is giving the same gas flow as a block that lines up.

But I am interested in your experiments with different carbine buffer weights. My testing with buffer weight was counter intuitive. Up to a point increasing buffer UNSPRUNG weight took less gas to run.

Let us know what happens and document as best you can. This stuff is fun.

Thanks

You know I’ve tried to sort out what upper you have and if I’m reading right it’s a front sight and not just a gas block.

So if the FSB is not centered over the gas port and you opened the hole in the FSB then that means there’s a good possibility that you irons will be way off center if they even sight in.

What you should’ve done was send it back to the mfg. but now that you messed with it, I doubt they’ll fix it but they might.

And on top of all the help you’ve received here from John and MM, you want to buy junk from spikes to make it even more of an anomaly.

Good luck.

Dude, I can't help you if you're not paying attention.

Well, that was my concern with opening up the hole in the gas block, but I figured, what the hell. It was only a slight amount, the holes were mostly aligned to begin with, just off a tiny bit. In fact, I had no functional difference before and after working the hole out.

By the way, the hole in the gas block appears to have been just ever so slightly off center to begin with, so while the sight itself is perfectly straight, the hole was just a smidge out of whack.

I will likely end up drilling the gas port hole in the barrel a little larger anyway, as JohnBurns and Montana Marine suggested, but I figured I could give this a try with the buffers. Nothing permanent there, as it's easily swapped back to the way it was before.

Thank you all for your help!

Keep us updated.
This is interesting

I'm going ahead slowly with drilling the gas port ever larger. I know, I could just hammer down with a .105" hole or something, but I'd rather ease up to it.

Drilled to .098". No appreciable difference in performance. Winchester M193 worked perfectly, as it always had, Everything else, still slipping over the magazine some times, and not locking open.

Drilled to .099". Now, all of a sudden, I see some noticeable improvement. I'm only firing a few shots at a time with different ammo to see what it does. With all ammo, ejection was more energetic, and much more like what I would expect it to be. All 3 to 4 o'clock, as before, but at least a foot farther away. And still very consistent. For the first time, I got through five shots of steel Wolf without it jamming or slipping over. Still no bolt hold open. Same with Armscor USA, and something else, maybe, I don't remember. But.. now was the very first time I got through with PMC Bronze, no magazine overrides, no jams, AND the bolt finally locked back when empty! That alone made my little clogged heart pitter patter! M193 operates just the same as before, but with the same improved ejection distance.

I'm not super worried about getting any of that steel case crap to work reliably, I mean, it's not like it's readily available anymore (and probably never will be), but I would really like to be able to reliably shoot the other normal brass case stuff. I'm not always gonna be able to be picky about ammo, so the gun can't be picky either. I think I'd rather get it going this way first, rather than go down with different buffers, and still, if necessary, I could use different buffers to tone it back down if I need to.

I will drill it out again to the next bit size, which, I think, is .101", but I don't feel like taking the thing all apart again right now... it's supper time!

I would be very interested to know what some of the other manufacturers, like PSA, Anderson, whatever, use for their "budget" (that dirty word) 20" rifles that are so horribly "overgassed". Just to see how I'm comparing.

Glad you decided against playing with carbine buffers. But I will say if you want to experiment just take a weight or two out of the rifle buffer. You should replace them with some kind of spacer but leave enough room for them to slide back and forth.

FWIW the rifle buffer weighs 5 oz and a standard carbine buffer weighs 3 oz.

Bitching about being overgassed has become the cool kid thing to do.


If one can't have a perfect gun🤣🤣🤣, I'd rather have one that will beat itself to death overthousands of rounds.


That's better than a POS that doesn't work.

+1

Ok.... I was kind of hoping that one more size up would do it, so, I drilled the gas port today out to .101" (a #38 bit).

Results after firing only a few rounds of each ammo type a few times

PMC Bronze 223: Fires and feeds, locks open
Armscor USA 223: Fires and feeds, but after one failure to lock the bolt open, it now works perfectly fine.
Wolf Military Classic 223 steel: Fires and feeds, and bolt locks open!!!
Winchester m193: Fully functional, bolt lock open, of course, as it always did.

A vast improvement over every other shot slipping over and "click", along with no bolt locks!

All eject very consistently about the 4 o'clock direction, and land 8' to 10' away (the M193 at the farthest distance, but still the exact same direction). Probably 3 feet farther away than originally.

Action feels more "snappy" but not at all "violent" (ever shot a Mini-14? There's a violent action). I don't really notice any difference in feel among the different ammos, but I wasn't really paying attention to that, mostly watching my brass and bolt!

I think I'm gonna stop here. At least for now. And shoot the thing a while. If problems persist, then I might go one more size up, but I think I'm good.


I'm feeling a lot better now.

Good to hear, might just want to confirm in some colder weather just to be safe.

MM

I'll have to wait quite a while before I can do that!