I spent a year w the Valkyrie and I reload. While I only used it for hunting, I can say these things about the cartridge and long-range shooting and feel like I have some experience to back them up.
1) the twist rate on a lot of barrels, including some I have, was 1:8. Bullet stability calculators indicate that heavier bullets needed at least 1:7.7, and a few barrels were offered w 1:7.5. A guy I know owns a gun shop and had FITS getting a new build to group worth spit w 88-90gr bullets but grouped fantastic w 60 and 75gr. I told him 1:8 was too slow but he refused to listen. He swapped barrels and got it good enough to deliver to the customer...still makes me wonder about 1:8 twist, especially at the lower MV that cartridge has with the 88-90gr bullets.
2) high-for-weight/caliber BC is cute, but a heavier/larger high BC bullet will group better due to being less effected by wind/air currents.
I got sick of messing w carrying 60gr rounds for long-range (hunting...300-400yds w thermal) coyotes and 88gr for pigs. Too much hassle planning or changing mags. 64gr Nosler BSB seemed the answer. Hahaha. No. I finally got a Grendel and haven't looked back. 100gr ELD-M, 2,750FPS MV, 0.365 (or 0.385, can't recall for sure) BC. One bullet, one load for pigs, coyotes, and Bobcats at any range I'll shoot at.
Finally, relevant to the OP: I've not seen any MV (or load data yet) for the 6mm ARC other than some factory ammo info maybe. The Grendel will support 123gr ELD-M for sure (Hdy Black), but I think the max bullet weight for any available 6mm bullet will be 115gr. That's close. *If* you could find load data to show that the 115gr projectiles could be loaded in the 6mm ARC without killing powder room, then that might be a better choice as *maybe* the 115gr (or similar weight bullets) will have higher BC than the 123gr 0.264" bullets. But, the MV will have to be considered, too. Until load data is well documented, I think it will be hard to make an objective decision and choice.
FWIW, I dreamed and drooled about a 6mm cartridge for the AR-15 platform for a year (the whole time I was pushing marbles uphill w the Valkyrie) but the lack of SAAMI standard cartidges held me back. I reamed my own 24 Nosler barrel and bought a 6mm Predator (super nice guy to deal with selling/making those) but all after I'd chosen the Grendel as my daily cartridge. Now, w the 6mm ARC, a standardized cartridge, available...well, I found this post while seeking more info on it. And I've had a reamer on order from PTG for at least 6 weeks. That said to say this: I love the 6.5G...but the 6mm ARC could be what I've wanted all along.
HTH, YMMV, EIEIO.
--HC
