Maybe I missed something but the Howa Mini is already produced chambered in the 6 ARC. How is bolt face size an issue?
He wants to rebarrel a 223…
That's why I suggested the 6mm Max, just swap the barrel. It should work with the magazine the OP has already as well. It isn't better than the ARC, just simpler to accomplish.
I said it was a flawed cartridge, the case is too long and you run out of seating depth quickly (2.30" COAL for 90 gr ELDX) in most magazines. Especially when limited to AR mags at 2.26", you should have a bit more room in a BA rifle. The ARC is simply more flexible with a wider array of bullet options. However, I'm sure just about any bullet between 80 and 100 grains in a Max would end a pig as easily as in the ARC.
Reloading info for the Max is available at BC Precision's web page, dies and loaded ammo are supposed to be available in November. It is a SAAMI cartridge, Starline is supposed to make brass as well. As was implied, this is all Googled information.
I'm sure the .300 Hamr would work as well. I like the .30O BLK, but I have a suppressor and love shooting subsonic bullets. Having a suppressor and a Rem 788 in .30-30 doesn't make the Hamr appealing to me.