The limiting issue of Win M70 Long Action is the availability of DBM bottom metal. If you got that figured or you don't want that and your cartridge of choice requires a long action, then build on it. If not, leave it as-is.
As messed up as this is, it's actually not unsafe. These magnum cases headspace on the belt, so the headspace is safe and the 300WinMag case is long enough the case neck is in the chamber neck of the 300Wby. Certainly NOT recommended, but not unsafe either.
In the early 80's commonplace to see long guns in the high school parking lot. In the 70's we used to bring our long guns and store them in the principal's office. Picked them up as we left and did some hunting on the walk home. Those were the days....
My 0.02 is a compilation of all these issues occurring:
-I think the chamber is not concentric.
-The press is at it's limit and not rigid enough to maintain alignment.
-The press does not support the die and the die is thin walled to begin with.
-Might be other shellholder issues.
Any single...
Some guys are running Major9 and/or custom minimum spec chambers. Those benefit from an small base die. If you have a factory chamber then it's no benefit. It won't hurt, just doesn't help.
Perhaps a .284Win or 6.5-284 would get you there, running 2.950" COAL. I was able to get .284Win - 2,800fps with 175's with a 22" barrel without any difficulty.
I haven't read through the entire thread but if this has been covered.
If you're looking for a magfed General Purpose load that can go to 1Kyds then the 175's or the new Berger 168's would be my recommendation. If you're looking for a magfed more dedicated long range load then I'd lean heavily...
If you want to load accurate high volume ammo this is what I have found to work:
Select components that are forgiving: bullet that is jump tolerant, powder that throws consistently in your powder measure, primer and brass that is reasonably consistent.
Old fashion load development to find the...
Depending on how much neck tension there was to begin with, it might be OK to reseat the bullet. If the bullet seats too easily, then you can run the brass part way into the sizing die to just partial neck size to re-establish neck tension.