Re: .375 Cheytac Action
Your action choice will be driven by the intended purpose of the rifle. Actions in the .408 bolt face are available from a few manufacturers in configurations from 1.450" to 2.0" dia, and in single shot or repeater style.
BAT, Stiller, Hall, Barnard, Lawton 8000, are a few that come to mind. BAT does not make a repeater action, but will lead the heap in quality...
If this is your first build on the .408 case, I strongly suggest you get with a reputable builder to walk you through he process. I define "reputable" as one who has built more than a few of these caliber rifles. There's a lot more involved to putting the right combination of parts together than reading information on the inturdnet...
We have a few sponsors here that are well qualified in this area.
Allen Precision comes to mind....
"Character is who you are when nobody is watching"