The 1-9 will stabilize the 300 grain SMK better at extreme long ranges (1 mile plus) a little better than a 1-10. From talking with Tim North and Broughton barrels he would recommend a 1-9 over a 1-10 for the big 338 cal rifles.
I have three barrels for the 330/408 all are 1-9s, they all seem to be pretty uniform in velocity. One of them is a little behind the other two in the accuracy department.
1-9" Broughton 5C buy it and let your barrel woes go away.
I had the 5C on my 6-284 which everyone knows is a barrel burner. Me and Kevin Bore scoped it after 400 rounds and I was amazed how little throat erosion there was. Took a new measurement and I was even more amazed.
It sold me on Broughton...Unless something happens to them you will never see another barrel on one of my guns.