factory ammo for one
and to be fair what are its disadvantages over a 6br?
In a bolt gun? Lack of quality factory brass is a big one.
There are no factory offerings from Lapua, Norma, Peterson, etc. for 6mm ARC brass.
The BR is also easier to tune, partly because of the quality and uniformity of the available brass, and partly because of cartridge design. The BR really is just incredibly easy to load for. And it has about a 10-12% advantage in case capacity over the ARC, allowing it to be pushed up into the 2800 fps node that a lot of guys find works well for bullets like the 105 Hybrid. Hornady does say you can get there in an ARC bolt gun with something like LEVER, but not with the super consistent and temp stable extruded powders the BR likes (such as Varget).
There are also a ton of options for 6mm BR chambers, allowing you to essentially build a barrel around the bullet of your choice. The ARC currently has one option, the SAAMI chamber that's optimized for Hornady's 108 ELD.
Other than that, you're essentially back to the old 6mm PPC vs 6mm BR argument (where the BR wins at 300yds and beyond), except again without an option for high quality brass. There are at least some high quality options for PPC cases.
And yes I realize you can make ARC brass from something like Lapua Grendel cases. But if the argument for the ARC is "factory ammo" certainly having to treat it like a wildcat to get quality brass counts as a mark against it.
Don't get me wrong, I'm not trashing the ARC. I have a 6mm ARC AR built on a Proof barrel and I love it, but the cartridge is really nothing special when you remove the requirement to fit inside of an AR15 magazine/receiver.
And factory BR ammo is available, from Lapua, for about $2 per round. That's compared to what, $1.65 per round for 6mm ARC Match from Hornady?
