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I have this dissipater upper and it has been giving me problems. If it's wet and clean it runs like a top, but the second it get a bit dirty is starts short stroking and wont load rounds out of the mag. It won't get through a full qual without having to open it up and slather it down with lube to get it to run.

So what do I do to fix it? lighter spring? lighter buffer? ????

chambered as 223 or 556?
what are you running in it?

rifle length gas? (I ask because some are middies) what brand barrel is it? what ammo?

New gun? Some of the lesser quality builds need "break in" time. You might change the buffer, but I'd explore other options before going that route. Gas guns, especially those turned out recently (to fill the sudden "need") may have problems with everything from port size to loose gas keys to mis-aligned gas ports. Hard to tell over the internet. Bring it out to my Saturday evening match at the Practical bays at BASF on 15 Nov if haven't figured it out by than, and I'll take a look at it.

L. Mike

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I had one that was doing kinda the same thing. I lubed up the buffer tube a little and that fixed it.

I'll tear it down and see if I can provide the info requested. The upper is pretty new but the lower is well used. The buffer tube and stock assembly are metal, not plastic. Every other upper I've put on this lower worked fine. I've also had dissipaters before that ran with no problem. Something is screwy with this upper.

Just based on description I would guess it's a gas problem. The gas key on the bcg may be loose, the gas block or tube may be dirty but being ever like you say I would venture to guess it's a bcg issue. As I pm'd before just hit me up if u want to head out to miller. I have a few ars and extra bolts we can move around to determine the issue. If you go out load 1 round in a mag and fire. If the bolt holds open and the shell stays in the breach area, it's the extractor. If the bolt won't hold open its gas, probably gas key.

Looks like the gas key is staked on and is tight.

I think we can rule out springs and buffer, because it runs fine whwn wet and clean. It could be something is heating up and with the dirt is throwing it out of specs. It will run after he puts more lube and this gives it time to cool. Also, it maybe the chamber is out of specs. causing excessive headspace or heating up making the case to stick. Just another idea to think about.

by the way, we are using XM-something or another, it's US military 5.56 we get on state contract. I don't know if it is more powerful or more dirty than commercial ammo.

Do you know the gas port diameter? If you have something less than 110" to .105", you aren't gassing it enough to get it to cycle when dirty.

I have to pull off the front sight to measure the gas hole, right?

Yes

try running it with 223 ammo (not the 556) and see what happens.