I have a 11.5" SBR that stovepipes every 40-50 rounds or so when suppressed (AAC M4-2000). It also does not lock open on an empty magazine reliably.

I have replaced the action spring, extractor spring, gas rings, and is dripping with SLIP 2000 EWL. I have also swapped out the H2 buffer for an H buffer and that did not fix the issue.

I have about 7500 rounds down the pipe and I've never had any malfunctions unsuppressed. The rifle was running reliably suppressed for about 2500 rounds with a form 1 308 can and then another 1500 rounds with the M4-2000

Should I try a standard carbine buffer? It seems like its short stroking but I don't know how that would be possible especially with a can. Maybe jump up to an H3?

I have not replaced the ejector spring in the bolt. Maybe that's part of the issue.

As a side note, I put the upper on an M16 lower (H2 buffer) and the ROF was really low....  

I contacted the manufacturer and they want $100 to inspect the upper.....

Start with extractor spring and potentially new extractor.  Gas rings good?  Does the rifle still run correctly un-suppressed?

Gas rings have been replaced multiple times and are good to go. The extractor springs are also good to go and the extractor was also replaced.

The rifle does still function reliably unsuppressed.

I find it very hard to believe that it is somehow now undergassed, unless you changed something in the gas system.

What makes the most sense, is that the springs in the gun started wearing, so you're getting higher cycling speeds.  What doesn't make sense is the slow ROF on the FA lower (I'll be honest, I don't have a lot of experience in this area).

Does the gun still work fine, even now, without the suppressor attached?  Did you fire this upper on the FA lower without the suppressor attached?

Have you checked the gas key on the BCG?  Possible you broke one of the bolts or they backed out and you're having a gas leak from there?  Unless you have an adjustable GB, that would seem to be the most likely point of failure in the gas system on a previously functioning gun.

The rifle functioned fine unsuppressed SA and FA. What I'm thinking is after 7500 rounds the gas port has slowly started to erode opening up the port slightly causing it to be overgassed.

Dano523 replied in another thread recommending a few things including trying out an H3 buffer to slow down the BCG. I'll be replacing the ejector spring and checking the gas rings, extractor, and extractor springs once more and Friday I'll be able to do some testing.

I'll report back what I find.

ETA: Gas key and gas block are good to go. I even swapped carriers to make sure. Also, the gas block is pinned.

The entire upper is factory and I've never messed with it.

I just had a light bulb go off..... Way back when I had less than 1000 rounds down the pipe, I ran some independence ammo through my rifle and had a couple blown primers. I just assumed I had some bad ammo but I've heard that is a symptom of an overgassed rifle.

Well, it does sound like an overgassing issue then, and port erosion is a very real possibility at 7500 rounds.  I'm still confused as to an overgassed upper causing a slower FA rate of fire on an RR (like I said, I have NO experience with diagnosing FA stuff).  

Either way, swapping to an H3 is an easy solution to try, so hopefully it works out for you.

Fast cycling gun with a magazine that can't keep up. If you aren't using a new mag from a good manufactuer throw that in the mix too along with the heavier buffer.

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What bcg you running?

I fixed the issue.... mostly.

So the other night I rigged up an H3 buffer (using tungsten weights from an H and H2) and test fired a few magazines without any issues suppressed. I then loaded up 10 magazines with just one round in each and the BCG locked back every time.

The ejection pattern has moved from 12:30 to now 1:30-2:00. I was firing M193 spec ammunition and also have a brand new action spring.

I'm considering picking up a few extra power action springs to move the ejection pattern more towards 3:00. What I've heard, is that an overgassed rifle (even slightly overgassed) wears parts out quicker than a properly gassed rifle. Is this true?

Would you all just leave it as is now that it's functioning just fine

I'll be out again later today to make sure it functions unsupressed 100% as well.

Thanks for everyone's help!

You are overgassed.  It happened to me with my 11.5.  I am waiting for some machine time to make my gas block adjustable.

MAHA

Sounds like you have it mostly figured out. What magazines are you using?

We need to have a stickied thread: "Suppressed SBR cycling issues? You may be over-gassed."

The issue seems to come up often enough.

You really should consider an adjustible gas block. The Micromoa Govnah is a great option.

I mostly use GI magazines with magpul followers but sometimes use pmags. To check function, I used 10 brand new pmags.

Meh. I run a Saker 762 with a 16" middy in .223. I just run a SLR Sentry. Adjust it to lock back with no can and run it with a can.

I switch back and forth a lot. I just leave it at one setting.

On a side note, I run a 1oz buffer and low mass carrier. The BCG never out runs the magazine stack. I would suggest having M4 feed ramps just in case.