I have a .300H&H in a Rem 721 and found a take off barrel for a Rem 700 chambered to .300 Win mag. If the barrel for the Rem 700 will fit the Rem 721 (I can adjust the headspace) will a .300 Weatherby reamer clean up a .300 Win Mag chamber?

I will retain the original .300 H&H barrel as is in case I ever want to sell it.

A 300 Wby reamer should just clean up a 300 Win chamber but.....there are tolerances. What if the 300 Win reamer used to cut the chamber was a hair on the large side and the 300 Wby reamer was on the smaller side of tolerances. I think you may have to chamber it and try it.

Properly loaded a 300 H&H will come within 100 FPS or less of a 300 WBY, so I sold my WBY..It feeds better, but most of all it extracts better when things go south. Its the most overlooked magnum out there and so much BS has been accepted about it that some experts just repeat what they have read by numbskulls who have never loaded for it or shot it..