Looking at duping factory contour to the end of the stock does anyone have one handy the could measure at that area?
300 wsm (8400 Montana) is 0.69 where it exits the stock tapering to 0.56 at the muzzle.
375 h&h (Talkeetna) is 0.81 where it exits the stock tapering to 0.70 at the muzzle. Both of those are 24” tubes.
I don’t have a 300WM or 338 WM to measure but I suspect that despite being 26” they are very similar to the wsm as they accept the same 0.56 factory muzzle brake kimber sells.