So -  I rebarreled my competition/DMR gun.   Compass Lake Criterion 1:8, 18", rifle length gas system.

Took it out yesterday, and it shot great.  Pounded 5 77smk's into .6" twice in a row.  

Today I went back to try a couple other loads, and after about four 5 shot groups (leisurely pace), the recoil became noticeably sharper.

The point of impact dropped about 2.5", and the accuracy went from subMOA to about 3".   Even Federal GMM, shot as a control group.  The same load that shot .6 yesterday is now 2.5".  

Took it home, and disassembled it.  Gas tube is fine, gas block looks fine, and I didn't measure it, but the gas port doesn't seem unusual.  

Scope is a NF 5.5-22x on arms rings.  All solid, no issue.  

Recoil spring seems fine too.

I gave it a good cleaning all around, but what the fuck could cause this???  

Am I missing anything?

What does the bolt and cam pin look like?  It sounds like either your loads are loaded with a different powder, or your rifle is acting more like a blow back.  Did it start throwing the brass to a different location/distance?

They were still dirty from yesterday, but nothing notable.

Good question - the brass WAS in a different location.   Yesterday, 2:00.   Today, 4:00.  

Is there any chance you sheared the locating pin on the barrel extension, or damaged the notch for it in the upper when you swapped barrels?  Did you hold the upper by the barrel or by the upper when you torqued the barrel nut?

This is a weird one.

I assume you checked the bore and chamber, cleaned them, didn't see anything wrong.

Gas block, tube, and gas port appear unchanged. We can eliminate overgassing I guess.  

I assume the barrel nut was tight.

I assume the recoil spring was the proper length? Carbine somewhere around 11", rifle around 12.5"

Weights are moving freely in the buffer?

Is it possible the barrel extension came loose?  If it was clocked slightly, this would make it more difficult for the bolt to unlock.

I'm running out of ideas.

I will have to disassemble and check the pin.   It was torqued using traditional vise blocks.  I'm not an overtorquer.   I can say the gas tube is still floating freely into the receiver so the nut hasn't moved.

This is a rebarrel of an otherwise excellent shooting rifle.  

A clocked/loose barrel extension is about the only thing that makes sense.  A gas obstruction would reduce recoil.  Unless something went horribly wrong, the gas port isn't going to erode significantly over 100-200 rounds.

The only other thing it could be is a bad buffer spring. That doesn't really impact accuracy as bad as you describe.

Thanks,

I just pulled it apart and the pin and slot both look just fine.  

It did go into this particular upper very tightly.   I changed to a different upper and it went in with what I would deem a more appropriate fit.  

I will see how it does tomorrow and report back.  

Yeah, fucking bizarre.  

I appreciate all of you looking at the issue.  I try to help out here where I can, and it really does come back around.  

Could the buffer be binding in the tube?

No, it's certainly smacking the back of the tube hard.   I know the difference.

Talked to CLE.   They feel there might be a throat obstruction of some kind.  I'm sending it back for them to examine it and either repair/replace.