I had an extra Ruger 375 rechambered in 375 Ultra a few years ago and it was straightforward as the reamer goes but you have to have the rails worked on because of the fatter cartridge. The only thing missing is a fat and concave bottom for added cartridge capacity. Recoil is a bit fast, almost approaching the Rigby's. The only reason I did it was I was thoroughly impressed with Jon Sundra's wildcatting the then new 300 Ultra to 375 quite a few years back. JS devoted an entire chapter in using it for an African hunt.