Been loading for a little bit for pistol. Going to start loading rifle so I can feed my expensive 300 rum. Got a set of neck size dies for it. Neck sizing once fired cases from my rifle makes sense for better accuracy. But how do I set up factory fresh brass?
If I work up a load using my fired neck sized cases, wouldn't that load group differently using factory sized brass?
I'd just shoot the factory brass as-is. If you only have neck sizing dies you will likely run into a problem with brass that chambers tightly after very few reloads. Especially if you load the RUM at its potential.
I usually find that big cases like the 300 rum and many magnums shoot about 50 fps slower when loaded virgin compared to the exact same load in fireformed brass. Depending on the gun that can be significant....or not at all.
I don't spend alot of time running virgin brass through any sizing die. You won't change much about the brass because it is already at minimum spec. You will get allthe neck tensions a bit more consistant.
All I do with virgin brass is chamfer the case mouth with a low angle deburrer, run a bore brush on a drill into the case neck to remove the "scale" that's usually there from the last factory cleaning of the brass, and plunge them over a lyman m die.
The rest of the good prep work for me comes after the first firing.