Need Barrel Help for 338-378 Weatherby Magnum.

[NorthernWeatherby]

I am building, a .338-378 Weatherby Magnum on a Mark V action from a .378WBY Mag. I am going to have Ron Smith In Alberta make the barrel. He made the barrel for my .300WBY Mag. and it is very accurate. I am not sure of the twist rate though. I am thinking of 30" Gain twist, ending out at 1 in 9. Any suggestions?????gun)

Northern Weatherby
.300 WBY Mag Custom.
.338-378 WBY Mag Custom (Soon to Be)