Over-gassing/Timing Issues

[notagrendel]

Thinking cap time. This maybe a post for the reloading forums.

I am having issues with belting in my 6mm ARC handloads and I am pretty sure it is timing related, but not sure completely. The combo is a 108gr Berger Elite hunter on top of 26.5gr of Varget using a Federal GM205M primer. Starline Once fired 6.5 Grendel formed at 1.182" HS (B350). COAL is 2.280". Velocity is at 2445 fps.

Here is my current built:
Anderson RF-85 upper
Proof Research 18" barrel in 6mm ARC
Rifle +1 gas system w/ SA AGB
JP Enhanced 6.5 Grendel bolt
Brownell's (I think) phosphate carrier
A2 rifle buffer spring and tube
A2 rifle buffer: replaced the aluminum spacers with steel weights from a standard carbine buffer (estimated total weight around 7.5 oz.). I will verify the weight tonight.
ASC 6mm ARC 5-rd. magazine.
Custom 6mm silencer.

When utilizing this load with 6 clicks from closed of the AGB, I did not have any belting*, the action cycled, but the bolt did not lock back. 7 clicks the bolt locked back and there was a slight noticeable belt that I can barely feel with my fingernail, but could not see with a naked eye. 8 clicks the bolt locked back and I noticed the belting occurred maybe a few thousandths up the case.  At this time I did not inspect the cases, for whatever reason.

Two days later I utilized that same load with 20 rounds with 7 clicks from closed on the the AGB. Half of them had belting on them.

Here are the big questions: Is my extra heavy buffer causing timing issues? Is the AGB opened up too much causing the case to begin extraction before pressures drop in order to compensate for the bolt not locking back?

*The way I measure belting is measured in feel and measurement. Belting seems to begin occurring at the webbing measuring 0.4405-0.441"

Is this what you refer to as belting?

That's what the factory ammo looks like, but my handloads have a rim at the very base of the body, yet pressure signs of primer flattening, ejector marks and extractor marks are more present on the factory than my handloads.

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...you can also try firing one of those rounds with your gas closed off completely and see if you get that "belt" condition on the brass.  I would also recommend you use a standard buffer to start out with....always a good idea to start with standard "baseline"  parts so you can determine what, if anything, needs to be adjusted and which direction that adjustement needs to go.

I just got back today from the range, and I tested out some loads that I thought weren't belting when I worked them up. They belted with the AGB closed to 1 click past LRBHO.

I'm by no means an expert ... but, could it be you have too much headspace? My fired brass measures 1.190-1.191 and resize to 1.188 or a half thou under that. Your around .008-.009" of headspace if your chamber is close to what I've seen common for dimensions. That would be working the brass pretty hard and don't think it would last as long as most before signs of case separation. My novice opinion should be taken with a chunk of rock salt.

My chamber HS is at 1.1855"-1.186", so 1.181" is only 0.004"