Spikes kit. Definitely >5k rounds through it. Went to lube it and noticed it's crowning.

Is the carrier slightly too long?  Is the upper hard to close with the carrier and bolt installed?

I like your buffer, btw.

Upper closes nicely. I can just fit a 5thou feeler gauge behind the carrier while closing.

I've got the plastic buffer, the ball detent mid, and, when I feel like it, I make plastic collars too. Definitely keeps the action quieter and delays opening to help with the integral suppression.

Idk. I went through it and see nothing amiss. No wear/scratch marks anywhere inside the upper. I figure the CG is above center so it could be flexing when contacting the buffer.

That's a good guess.

I used a similar buffer screwed or glued to the frame for maybe 10,000 rounds, but I also had a delrin rod in the receiver extension that used a bolt to adjust the length so the back plate was supported.  I was concerned about the back plate and fork bending in the other direction since the top would have been supported by the lower, but the bottom of the plate would lack any support with an empty extension.

That looks like an older (pre-2013 or so) CMMG stainless steel frame (no notch for the forward assist lug).  In any case, what you have experienced is age related creep (or 'time warp', if you will).  I have a 2010 vintage CMMG stainless steel frame that by 2017 had warped so much, the bolt didn't fit in the upper any longer.  I contacted CMMG with the photo showing the creep issue and they sent me a new frame free of any charge.

Prior to contacting CMMG, I did try, without success, to bend the warped frame back into the correct position. The effort only resulted in my referencing the part as having had an incestuous relationship with its female parent.

MHO, YMMV, etc.

Makes sense. Unfortunately, I have a folding stock. I'm sure I could figure something, but there's no real issue yet so I'll roll on.

@SecondAmend
Good info to have. Thx

I don't think it's an issue with the length. Regardless of length, the carrier is rest against a spring loaded stop.

The way yours is acting, it makes me wonder if yours is made of a different material from mine. On mine, the back plate cracked after about 10k rounds. CMMG made it right by sending me a new rail.  That one cracked after 10k rounds or so.  I just kept the first one and welded it back together again. That one has seen a few cases of ammo and is still going strong.