I recently built a 300 blk upper and took it to a local Smith to to the headspace check and give it a once over.

Took it out and started shooting and it cycled fine. Did about 15 rounds no issue trying to fine tune my scope. When I picked up my brass I notices a flat spot on the neck. Never seen that before.

I have an adjustable gas block that was wide open. I read about how to tune it so it is optimized. This involved taking it down until a round did not eject and then opening back up some. I did this and fired 3-5 more shots. Everything cycled and this flat spot was gone on the brass.

I then loaded up some mags and moved out to 100 yards. At this point Everything when to crap. I had a lot of failure to eject and some stovepipe. Reset the mag, opened the gas block, pulled the BCG, checked things were tight. Tried again and the same issues.

At that point I stopped messing with it. Anything else I should look at or should I have a gunsmith troubleshoot it?

All with the same ammo?

At the end, how much did you open the gas?

If it were me, id make sure she's well lubed and start from scratch on setting the block.

Flat spot on the neck is just from the case hitting the brass deflector on the way out. Turning down the gas made it not get ejected as fast. No big deal unless you reload but even then the ding usually isn't so bad it can't be resized.

What changed when you moved back to 100 yards? Were the magazines fully loaded vs. partially loaded, suppressor, resting on mag, ammunition? Is the gas block still in place/on tight?

You may check to see if the gas block moved any. I've had that happen on gas blocks that are not staked or held by pins. This will result in the gas port being fully or partially obstructed and cause malfunctions.

Something changed between distances on your gun. Assuming you are using the same ammo, there are a couple possibilities. The gas block may have walked, the adjustment may have tightened a bit, etc. Pretty common on a new build, so don't fret. Just need to shake the gremlins out.

It seems you are already doing more than the average ar shooter in learning to adjust the gas, so I'd first look to see if the gas block looks like it has moved forward a bit or if it has rotated to the side.

If it seems like it hasn't moved, I'd start looking at the gas system. Most adjustable gas blocks have a screw for adjusting the flow and another one to lock down the adjustment screw. If you didn't lock it back down, just the vibration from shooting it MIGHT have caused it to move a click or two. Unlikely, but I've seen it happen before.

Another thing, are you shooting suppressed? If so, you're getting a lot of carbon blown back into the system. I don't think that a few shots should give your gun a coronary, but it is possible. May just need to clean the gas system out.

Thanks for the replies. I will try and answer all the questions.

All the same ammo.
Not suppressed.
The barrel was dimpled by the gunsmith when they checked headspace. So that should help hold the gas block in place. I didn't notice it had moved or was loose when I was adjusting it.
I loaded about 10 rounds or so when I was first shooting at 25 yd and zeroing the scope. Once I was pretty happy with it I loaded one round in the mag and adjusted the gas block down. Fired and then looked to see if the bolt was locked open. I kept doing that until the bolt didn't lock open. Then I opened the block back up a few clicks and verified the bolt locked open.
After that I moved to 100 yd. When I started having problems I went back to the original setup when I first started.

I pulled it out of the safe today. Didn't see anything visually. Wiped it down and put on a fresh coat of lube.

Going out to the lease this weekend and will take it with me.

You might also do a visual on the gas tube both at the block to make sure it is still pinned/aligned correctly and in the upper where it inserts into the gas key to make sure it isn't bent/clogged/otherwise damaged. Like EyeGuy said, it would seem to be an issue in the gas system somewhere if all else is the same? Hope you can get it worked out!

Have you tried tighter spandex?