Hello

I have an 11.5 inch AR pistol that is an FN CHF barrel and I cannot get it to cycle properly.

I have tried

Sprinco white plus H2 and H1 as well as carbine
Traditional spring with the same three as well.

I switched out the gas block to a traditional gas block as well.

It will not cycle steel or brass consistently.
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What exactly is it doing?

Short stroking. fail to extract, fail to eject, ?????

Single round fired, fail to lock back???

Gas port too small???

@DasRonin

It is ejecting and extracting around the 5-6 o clock position.  

I have not tested one round and seen if it will lock back.

I just tested one round in the chamber and the bcg did not go all the way back to the bolt catch.

Did you assemble the upper yourself?

Did you use a vise block?

Sounds pinched.

It's either that, or gas block misalignment.

Could also be a loose gas key on the bolt carrier.

What ammo?

Gas issue. If bolt.is not locking back it means there is not enough gas to push the bolt back enough to extract and chamber a new round. Could try carbine spring and buffer and do what others have said. Check gas block alignment and make sure port and tube are not obstructed. Worse comes to worse drill out the gas port one size bigger

I did assemble it myself I torqued everything to the mfg specs. I used a receiver block I did not vice block the receiver itself.

Ill check the alignment.

I tried it with a bcg that I know is legit and same problem.

For the single round test I used tula steel and federal m855

I tried a carbine buffer I am going to check the gas block.

I did a build last year with an FN 14.5 barrel and it did pretty much what you describe.  After experimenting with some spring / buffer combos, my smith torn it down, reamed the gas tube hole ever so slightly and we reassembled and now it functions 100%.  The most annoying part was re-installing the rail with the thread locker etc.  It is presently using a Geissele braided spring and H buffer.  YMMV.

I am going to check the gas block and if the issue persists then I may need to open the gas port slightly.

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.

If it won't lock back on an empty mag, you are either light on gas or heavy on mass, or both.  A heavy buffer is not the answer to that equation.  You need to lighten up the cycling mass or increase the gas, or both.  If you have an adjustable gas block, may as well open up the gas port.  You can always choke it back down.  Magazines can often be the cause of cycling issues, but shouldn't prevent bolt from locking back on last round.

Before I left the range today I swapped my upper onto my buddies lower which is an H buffer and a standard spring. I used one if my mags and it cycled tula steel like nothing. Even though I had tried that on my own lower.

So your lower has had the same H buffer and standard spring and when you pin your upper on your lower, with the same internals as his, it won't lock back on empty?

It will not. At this point im going to weigh my buffer and my friends and see if theybare actually the same.

Wonder if your bolt it riding on your hammer and slowing it down so it doesn’t function properly.

That's now my thought if the upper was tested as good.

Either that, or the lower is out of spec, which I highly, highly doubt.

OP, did you build out the lower?

Can you share pics?

Well that's odd it works on a different lower. I know you said the bolt doesn't lock back but what is the actual issue of cycling. I'm not sure if you answered that question.  You said it ejects fine so I'm assuming a double feed or failure to load? Short stroking could also be cause by weak ammo. Which you said you were using tulammo correct? I had one gun that one constantly short stroke and jam on almost every shot but another gun would run flawlessly with the same ammo. Trouble shooting this issue can be tedious. You are doing the right things. Something I noticed no one mention is also to check your bolt catch. Can you lock the bolt back manually when you engage the bolt catch?

You are right. It does happen sometimes although it is rare. Check the hammer  and the bottom of the bolt carrier for excessive wear. Even highly reputable company's can occasionally produce and out of spec product.  We really need pics to see what is going on here

I will takedown the gun later today and post pics.

@-Obsessed-


I have posted pics in the OP

I did a build with a nib FN barrel recently that was short stroking and my gunsmith ended up having to ream out the gas hole ever so slightly and then it functioned perfectly.  He put a steel rod down the barrel to prevent messing up the rifling and just used the first bit that was larger than the existing factory hole.

A ton of suggestions of things to check except what''s obviously wrong from the photos.

Gas block is seated against the handguard ring shoulder without a spacer. Gas block should be .03~.05 forward, likely solve your problems.

Your observation may be correct, but every gas block I have ever used has had a hole about twice the size of the gas port so a 32nd of an inch one way or the other won't matter.  But, it might be worth a try.

It does mattter, I've tested it. Even if the gas port isn't obstructed, being that much misaligned from the hole in the gas tube can and will cause function issues.

just fyi, in general, changing the mass of a reciprocating assembly that's got a spring to return it to position has zero effect on the distance it travels.  it changes the speed at which it moves.  since there is damping (friction) which is speed dependent, slowing down the speed the bcg goes backwards might have an effect on the energy lost due to friction, which would impact distance traveled.  but the distance traveled for a given amount of energy imparted to the bcg+buffer is determined by the spring rate of the buffer spring.  the equation is Ep=1/2kx^2 where Ep is the energy stored in the spring, k is the spring constant, and x is the distance displaced.  m or mass of the moving assembly isn't part of the equation.  Ep is basically the energy from the gas system less the energy lost due to friction as it moves back.  changing Ep by enlarging the gas port is a lot more difficult than changing k, the spring rate.

I mistakenly assembled a carbine gas 300blk 16" barrel instead of buying a pistol gas barrel.  everything was great except no bolt hold open on an empty mag.  bcg wasn't going back far enough.  swapped out the buffer spring for a reduced rate spring, works perfectly now.  no disassembling of the upper, no reaming the gas port.  took me about two minutes to accomplish with no tools.  problem was not enough energy coming back through the gas system to get the bcg moving backwards.  which is why everyone recommends pistol gas - you get an earlier and stronger pulse of gas to get the bcg moving.

if the problem follows the lower, not the upper, which it sounds like you're telling us, look at the lower.

When I disassembled it that was one of the first things I saw and thought maybe it was too close to the shoulder. I bought some gas block alignment pins from HB industries to try and get it centered.

My first thought after reading that the upper works on another lower is too much recoil spring. The recoil spring may literally be too long or too stiff. If you can get your upper on that same lower again, with no ammunition, carefully compare how much force it takes to draw back the charging handle against the spring. My guess is his will be easier than yours. Now, this doesn't mean something else may not be wrong, like a bad gas port or a gas leak. It could simply mean that his weak/worn spring is what you need to compensate for low gas. But, assuming no gas problem, it also could mean your spring is faulty or the wrong one.
I liked the comments about bad magazines and how they can be a 'drag':), but your mag worked in his lower. Again, that doesn't clear your lower because the mag catch area could be out of spec, holding the mag too high.

Interesting.  I haven't had any function issues with any of my builds.

never thought to use an old spring!  all my buffer springs were basically brand new at the time.  old spring would have lower spring rate, which would also lower the force it takes to move the bcg back from in battery.  

basically the same kind of discussion motorcycle racers have when setting spring rate and preload to get a specific static sag.  my racebike forks also had a hard limit on max compression under full braking - the front tire would rub against the front cylinder head cam cover.  I spent a bunch of time understanding the effects of cutting coils off a spring to increase its rate, and adjusting preload with spacers.

edited to add:

from the pics, looks like minimal or no grease on the buffer?  when I pull my (admittedly few and recently assembled, I'm not a gunsmith and never played one on tv) rifles apart, there is always some grease peeking out around the buffer.  if the rest of the rifle is also under lubed, the unnecessary friction losses will reduce the distance the bcg travels.  if everything is new, you're also wearing off all the high spots on the metal mating surfaces, which takes energy lubed or not.  I hand cycled my first two about 1000 times without lube, then stripped and cleaned out all the aluminum shavings and then lubed them.  the 5.56 carbine has always functioned perfectly.  the 300blk needed the reduced rate spring for bho, otherwise it was perfect function as well.  the Dolos setup I put together I hand cycled dry about 500 times dry, then cleaned and lubed, it's run perfectly with the 5.56 and the 300blk barrels, but the 300blk barrel is a pistol gas not carbine.  

OP, the process of elimination can start right there, can you - #1) replicate the function with your buddy's lower again to see if you can get the bolt to lock back on an empty magazine (1 round test)? if your upper works, then #2) borrow his lower's spring and buffer, try them in your lower to see if the upper would work? If yes, there is your issue, if not, then there is possibly an issue with the fire control group.

If your ejection pattern is 5-6 O'Clock your definitely under gassed/over sprung/weight. Even though it worked on your buddy's lower his spring/buffer weight may have been enough of a Band-Aide to cure your underlying problem. I'd start with the gas system. Remove the gas tube from the block, use an slightly smaller drill bit and hand turn it inside where the gas tube port is to check for any metal shavings from the manufacture. Check the gas hole one the barrel, it should be @ 0.076. Blow out the gas tube with compressed air and check for good air flow. Check the gas key for cracks, loose fasteners and the torq value @ 50-58 in/lbs. Use a pipe cleaner/weed wacker wire to check for obstructions in the gas key to carrier to be sure the pathway is clear. Check your bolt to see if all the gas rings are present. Measure the bolt tail average to see if its in spec. Also look at the bolt lugs and barrel extension for damage.

Check the size of the gas port!!!

I just went through everything in trying to solve a short stroking FN 14.5” barrel. After getting some pin gages and checking the port, it was severally undersized.

For an 11.5” barrel, you probably want to be around .070 at least.

Rifle spring in carbine receiver extension?

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80 Percent Arms   |   Oct 13th 2020. Had a really good write up for troubleshooting ar15 problems. And was very helpful getting my PSA 10.5 upper working with my built lower.

Before shotgunning it and swapping out parts or drilling out things, which may not be the problem but yet by doing that it may induce more problems and mask the actual problems. I would check and eliminate what “might” be causing the problem through some elimination process. You might have several issues, gas leakage, excessive friction or binding, tolerance stacking, etc. Think of these as links in a chain. One link may be under spec or at the low end and another link might be at the high end. Yet the links are each within the spec of being good. But when you try to put those two together they don’t fit.

Everyone had good suggestions and observations like gas keys leaking, gas tubes leaking, parts binding, etc. Always use good ammunition. Tula steel case is known to be inconsistent and low pressure. And steel case in a new ar15 chamber, let alone chrome lined may stick. What kind of carrier do you have?? AR15 profile or M16, the M16 carrier is heavier/more mass. I first would try and take the bolt carrier, check your bolt and gas rings, and turn the upper receiver upside down by itself. Then take the assembled bolt carrier and run it from its fully forward/locked position and see if the gas key is mating with the gas tube fully. Is your gas tube protruding enough into the receiver? See if the gas tube is bent a bit and causing binding and also that the bolt carrier travels rearward smoothly in the receiver without binding. Is the hammer somehow riding the carrier bottom causing friction? Maybe hammer is out of spec. You did state it functions fine on another lower. Does your lower have any type of tensioning screw to adjust upper to lower tightness?? Check to see if the bolt carrier sits correctly. If you have the correct spring and buffer length in the rifle for the length of the receiver extension, when you pull all the way back on the charging handle, the face of the bolt should stop about 1/8" to 1/4" in front of the back edge of the ejection port window.  Also, during the last inch'ish of the CH pull, the needed tension to pull the charging handle all the way back, should not increase with needed tension.  If you get increase tension during the last inch, then either the spring is the wrong length, or the hammer is wedging instead. Turns out one of my pistol builds had a problem with ejection exactly like yours. First, my gas tube was off just a bit and caused the gas key to bind on it. Next, when I measured the bcg bolt face at the rearward position it was off. Turns out my carbine buffer spring was off spec and too short and not enough coils. Then went with a spikes tactical T2 buffer and flat spring. No more problems and ejection dropped right where it was supposed to.

When I put it back together I moved the gas block forward per this thread. I just have had zero time to go back to the range.