Re: 300grn 338, seating depth?
I've only shot them in a .338 x .378 Wby, but the longest load I've tried is 4.00" OAL. That still leaves me with .115" of freebore in my factory dimension chamber. I don't intend to get any closer to the rifling than that. I have seen 1/2 moa groups in several custom Wby Boomers with factory magazine length ammo, so the 1/4"+ freebore that they're shooting through doesn't seem to hurt accuracy. The 300gr Smkhp's are too long to seat to mag length in my opinion and need to be single loaded. If you decide to shoot the 300 gr bullets in loads that exceed magazine length and in chambers that have a lot of freebore, you'll have a lot of room to experiment with oal. I wouldn't get closer than .100" off the rifling, but you should still be able to find a sweet spot.
Stay safe, Tom
Texas State Rifle Association Life Member
NRA Endowment Life Member
A big fast bullet will beat a little fast bullet every time