I own one rifle chambered in Rich Sherman's original 30-375 SI, which was the 375 Ruger case necked to .308, and 40 degree shoulder. That chamber was born about 13yrs ago.
Next I chambered a rifle based off the wildcatted 8x68S cartridge, in the effort to get higher quality brass. RWS and Norma brass was available for the 8x68S. The 30-8x68S improved has the same case capacity as the 30-375 Ruger improved, but a longer case head to shoulder length, since the 8x68S is a longer cartridge. I ordered a chamber reamer from Manson for my 30-8x68S improved.
Next I used my same Manson reamer to chamber a 3rd rifle in 30 Sherman Magnum. When the high quality 300 PRC cases became available from ADG, it was a logical progression. The Hornady manufactured 375 Ruger cases we used originally for the 30-375 SI were considered inconsistent and lacked case head strength. Rich and I were both seeking higher quality brass. That quality became readily available when ADG began manufacturing 300 PRC cases. I think Peterson also manufactures 300 PRC now also. Maybe even Lapua...
I could chamber a rifle for all 3 of my wildcatted cartridges using my Manson reamer without any significant handicapp. Just by running the reamer in to the appropriate headspace depth. That's how similar the cartridge dimensions are.
In fact, I use the same custom Hornady resizing die for all three rifles.
