With a 300 Win mag you definately are not going to increase or even match the maximum range of jacketed bullets with any cast design. None can match the jacked B.C.'s nor can any be driven as fast.
But cast does have one advantage jacked cannot give, and that is personal input, which makes the hunts memories much richer. Many archery hunters take up the bow for this reason, as do most handgunners.
I have killed an elk at 450 yards, several deer and similar sized animals at 300 yard, and my longest shot on a varmint was 575 yards, using a cast softnose bullet sent out from a 30-06. Some bullets I drove at full power, but settled on 2400 fps as being very adaquate and took the two longest shots above with that velocity. The key to making moderate velocity loads hit at long range is shoot the gun a lot, ( with only one load! ) at rocks etc at unknown ranges in the field, in all lighting conditions. The softnose will make them work at any range you can hit an animal, and a load that will kill a moose will take a mouse just fine. Little critters actually don't have to be hit with screaming vaporizers as the gun rags might lead one to think. Never ignore the old adage. Beware the man with only one gun. But understand that one load is as mandatory as the one gun to make a shooter deadly any time, every time. So don't take a light fast bullets for antelope and something heavier for elk, mulies or whatever.
If what I've said above fits your fancy, try cast. But purchase my book and read it carefully so you understand what you are doing.
If not, use Nozler partitions, handloaded to the wall, get a good feel of where the load hits, and use that one load for everything in the west.
For use with Lee sizers, I reccomend LBT blue soft lube applied before sizing.
