Hey guys! Hoping for some help here. I have a .204 ar that I built recently. I cannot get this thing to cycle to save my life! I’m using lancer mags and factory ammo. Almost every round when trying to chamber creates a “half moon” on the brass and dents the hell out of it. I’ve made sure the feed ramps are nice and polished and match up well, mag is fully seated. I’ve even tried a factory built lower and still have the same exact issue. Any thoughts as to why this happening?

Sort of sounds like an under gassed problem.  Is your load known to work in ARs?  Not all powders are compatible with ARs so start there.  If the powder and charge (even those factory loads) is/are known to work, check gas block alignment.  No blowing through it or other half-assed alignment tricks, take measurements and know, not guess, the GB is aligned correctly.
If that checks out, check gas key screws on the BCG.  Make sure the bolts are not broken.  Don’t forget to make sure the gas tube is properly installed in the block.

Next, what gas length?  So many people jump on this goofy +1, +2, or +3 length and it is not needed.  The only time I’ve used a + length tube caused me nothing but trouble.  Never again, no matter what the hive says.

One last thing, if you are using extra powered buffer springs or H buffers, go back to standard and see if it helps.

https://www.ar15.com/forums/AR-15/Troubleshooting/66/

Should feed just like 223. My 204 Shilen barrel feeds fine but was horribly over gassed. It required an adjustable gas block to stop ripping rims of the cases.

My brother and I both have White Oak Barreled 204 Ruger AR's. Both have always fed fine. No Magazine issues of any kind. We have always used either GI 20's or Brownell's version of the GI Magazine. Also a metal 10 rounder, not sure of the brand. Never a feeding issue.

Both are also badly over-gassed. I have an Adjustable Gas Block on mine, and we have an Adjustable Carrier Key on my brothers with a heavier bull barrel that is no longer offered.

Will it chamber a round manually dropping the bolt? If not that sounds like a Magazine or Barrel issue.

If it will feed the first round manually, but will not feed when firing it. That will probably be gas related.


Bob R

So typically if I just try manually cycling without firing, it will feed the first round fairly decent without denting the case too bad but when I try to manually eject and load another in it just half moons every case from there and the bolt won’t fully close. I’ve tried about 5 different mags all with similar results. Lancer, pmag, ar stoner, cmc and I think a colt. I’ve tried factory loads and hand loads. I had another .204 that I built awhile ago from the same manufacturer and barrel  and it cycled flawlessly. I just don’t know what to do with this thing!

Pictures of what is does to a case?

Kind of hard to see the severity of it… but it is definitely created a heck of a gouge/crease about 1/2” from the shoulder.

Still looks like a gas problem to me.  Stupid question, but are your gas rings intact?  If mine, I would start at the gas block and work my way back to the bolt, stopping at every joint along the way to measure, check, and double check everything to confirm it is all kosher and make damned sure a gas key bolt is not broken.***  Also, check the size of the gas port.  While you have the gas block clear of the gas port, make sure nothing is stuck in the gas port, both the barrel and the gas block.  Also check to make sure the gas tube is not kinked or plugged with something.

I know you said you tried a factory built lower, but the only thing I can think of in the lower that could cause this is using a rifle buffer in a CAR receiver extension.  It would be really queer if both your lowers had the same rifle buffer in a CAR RE.  If that were to be the case, twice, stop building ARs.  Not your forte.  

*** years ago I had a rifle that worked perfectly, suddenly started short stroking.  It turned out the forward gas key bolt was broken. It was staked in place securely so just looking it or using mild torque on the Allen wrench, it would not move. Only when I decided to completely remove the gas key to check both bolts did I find the front one was broken.  I replaced the bolts, restaked the key (no, I did not replace the key, just restaked in another spot; that is sure to cause some to have conniption fits), and it has never missed a beat since over a few thousand rounds.

So this is what it’s doing that is the problem causing the dent in the casing. Just so we are clear these are not being fired, this is just cycling the action by hand to see how it functions. Poorly. Obviously! Any ideas on how to fix?

Hellbender's mods

Lancer magazines have relatively short internal dimensions compared to G.I. aluminum. I would try different magazines to start with.

Are you sure the gas block is perfectly aligned with the gas port? This is usually the first thing to check on a home build, especially if it's not cycling properly. I use a pin gage or drill bit (the end that goes in the drill) to establish gas port dimensions on any new build and record it for future reference. Then I measure the distance from the barrels to the pin/drill bit with calipers and add one half the dimension of the pin/drill bit to establish the gas ports centerlines exact distance from the barrels shoulder.

Likewise, I measure the gas blocks measurement using the same method, using a pin/drill bit inserted through the set screw hole into the gas port hole to establish the centerline measurement from the gas block port to the gas blocks shoulder. The difference between these two measurements establishes the gap I need to establish between the gas block and the barrels shoulder to obtain perfect alignment.

With one round in the chamber and an empty magazine, will the rifle lock open after one round is fired? No rifle will function reliably if it can't pass that test.

Do you have the right buffer spring for your application? I had a friend having fits because he was using a rifle buffer in a carbine tube, which made it impossible for the bolt to travel all the way to the rear.

Are you using reloads or commercial ammo? For an AR-15 to run reliably you need at least .003" of headspace clearance between your ammo and the chamber. Anything less than that is asking for trouble, especially once the rifle gets dirty.

Who made the lower? Have you tried that lower with a .223/5.56mm upper?

A lot of people have had fitment issues with 80% lowers with magazines being too low or too high to feed reliably.