I've got one in my future just haven't pulled the trigger yet. I have a Beretta Mato 300 win mag which shoots the 200 gr Accubond at about 2900 fps and I would like to bump that up to 3000 fps. Plus just the fun of doing a wildcat. I have a 375 Ruger and brass.
Here is what I have discovered:
4D has a reamer for rent
4-D Products Rentals
The reamer should clean up a 300 win mag chamber and the barrel may have to be set back a thread or two.
The hard part was figuring out how to get the brass neck sized down. I called Hornady IIRC (might have been Redding) and there was one of their techs that shoots one and they recommended stepping down the brass with the following dies in sequence:
375 Ruger FL without expander
338 RCM FL die (neck sizer wouldn't work because of forcing cone internal slope)
300 RCM Neck Sizer
I got the dies and they do work, here are some pics with 375 Ruger and 300 win mag cases
The 30/375R neck resized case grew about .010" in length but no significant increase in neck thickness.
Of course, AFTER you have fire formed a case or two you can send them off to get whomever to produce dies; Lee Collet, Redding Body Die, et al
IMO it would be a gun you could shoot without a muzzle brake. It would essentially be a 300 win mag MAGNUM without a belt where you could have low headspace and not worry about case head separations. In my case where I have a 300 win mag and a Lothar Walther barrel existing on it, the price would be around $300 without the cost of a barrel or refinishing the gun (hopefully).
Just haven't got a