slick and meyer, is that two of you? just kidding. I like the project you're undertaking. Gutsy!
The 300 Win Mag is not the same for lead loading as 30-30, 30-06, .308, etc. This case was not designed for cast loads and I will depart from my friends here on this board in that Lyman manuals do not even come close to adequately addressing cast loads in the belted mag cases (even though I have several and use them extensively). That said, I will tell you what my experimentation with cast in the 7 Mag has done. First, the case is too big to burn powder efficiently. Second the neck is too short to efficiently hold a bullet. You also have to contend with bullet weight and design, type of lube, seating depth, barrel length and twist, among other things. I worked the powder rate chart from 7828 to Unique back to 7828 with and without over powder wads, LR and Mag primers. The resulting 'best' round was an RCBS 7mm-168-SP, checked with home made lube in all grooves, sized well over .285, seated to where the base of the bullet was at the bottom of the shoulder in a neck sized case, over 30 gr of IMR 3031 with a dacron over powder wad. Results? 5 rounds at fifty yards, .57 inch group (Rem 700 with 4X scope), repeatable. I guess the point of this is, that if you are going to shoot cast in the mag cases, it takes ALOT of experimenting with stuff that is not published (the stuff in the manuals is barely a starting point). btw, my bullets are made from range scrap and run about 14 to 15 BHN. My experimentation is ongoing, and I'm sure at some point I will find another 'good' round. I hope I don't sound too 'preachy', but expect to spend some time looking for a load that is both very accurate and very repeatable. As with all of my other cast loads, I do not clean the bore. No leading, no carbon. I attribute this to a combination of things, but mostly the lube. sundog