I've been shooting AR15s chambered for the 6RAT for several years, and finally got around to doing a bolt rifle for the RAT, using a CZ527 carbine in 7.62x39 to get the correct boltface. Using 6.5 Grendel brass, we neck it down with a 6RAT FL sizer, leaving a false shoulder for fire-forming, much like forming 6 Dasher by expanding the neck with a 6.5mm mandrel, then partial sizing it back down to 6mm, again leaving the false shoulder for fire forming - in fact, the finished 6RAT looks just like a miniature Dasher. Not at all sure how thick the neck would be using this process to form 22PPC Improved from 6.5 Grendel, but if I were going to shoot XC with a similar cartridge, I'd certainly go with an improved version to get increased case capacity. BR shooters are using something very similar to the 6RAT called the GRINCH, and having fine results where accuracy is concerned.
As an aside - now that Howa is offering their Mini action in 6.5 Grendel, we have another very good factory action available for not only the Grendel, but all its wildcat offspring. I've got a couple of these Mini bbl'd actions, and though I have no interest in building one for use in the OEM Hogue overmolded plastic stock, I just received a Pro Varmint stock from Boyds with the Mini inlet. Also have sent bbl'd Mini actions to Manners & McMillan, and am expecting stocks from them by SHOT or shortly thereafter. MDT also makes their LSS chassis for these actions. Neat thing about the Mini is that it comes with DBM (even though it's plastic), so it's not far from ready to go as a XC vehicle. Warne makes 20 MOA sloped scope rails for this action.
ETA - When I chamber an AR bbl for the RAT, I do so with the understanding that it's sold with a J-P Enhanced 6.5 Grendel bolt. Used to buy LMT 6.5 enhanced bolts, but when they discontinued them, I switched to the J-Ps. I feel these 9310 bolts give a substantial safety margin over bolts made of mil spec Carpenter steel.
