I have never loaded lead in a 300 Win Mag, but only because I have never owned a rifle in that caliber <BG>
I have loaded lead bullets in almost everything else tho- from 22 Hornet to 458 Win Mag, It sure beats doin crossword puzzles.
My old Lyman Cast Bullet Handbook gives some data for 300 Win Mag. One example is a 150 grain gas checked bullet with 22 grains of Unique. This should get you 2,000 fps at 37,000 CUP.
You see that small charges of fast powder is usual with lead bullets. One interesting thing to try when testing loads is to point the rifle up in the air after chambering a round- to settle the powder against the primer.
Try this for ten shots, then point the rifle down for ten more and see what happens to your velocities ( if using a chrono ) or your point of impact if not chronographing your loads. It makes for some head-scratchin <VBG>
Lay up some blackpowder and flints
The rest we can build, if need be