I have a 375 H&H built on a Vanguard action. My gunsmith's response when I asked him if it could be done was "they make a 300 Wby, which has the same COAL." Maybe so, but I ordered a 300 Wby mag box from Weatherby for mine (it was originally a 7mm Rem. Mag.), and it will only accept a maximum COAL of 3.530". The result? 260-gr. NABs have to be single loaded. To eject a loaded round, I have to move the bolt stop and partially to remove the bolt. Oh well, I figured it's not worth modifying the action bottom. 270-gr. TSXs feed fine, as do any 300 gr RNs. The NABs group into 0.6" with RL-15, incidentally...