Probably preaching to the choir to most of you, but with 300 WM, there are a couple of things you need to know before reloading it.
1. The 300WM is probably the only caliber out there where SAAMI standards are not held to by different manufacturers in the U.S.
2. A study of all belted magnums held the shoulder/datum line length to a minimum from the belt with the exception of the 300WM in both chambers, in brass, and reloading dies.
3. Chamber reamers from different sources can differ on the shoulder/datum line lengths and factory rifles can vary up to .035" to the datum line which makes reloading the 300WM a different story.
4. Brass is different as well, with .015" between different manufacturers. This has been offered as the reason 300WM chambers are so sloppy.
5. Reloading dies from different sources can be as varied as the brass on lengths.
6. Shooting rifles built in this manner often produces inconsistent case stretch, (datum line headspace length) shooting the same lot of ammunition in the same shooting session.
7. This produces inconsistent reloads, extreme deviations, and poor accuracy when not returning the datum lines to the same length when reloading by neck sizing only.
8. Full length resizing to return all the brass to the same datum line length overworks the brass and speeds up case head separations.
One US Customs' gunsmith spent a large number of dollars on reamer designs and came up with the 300WM reamer dimension the US Army AMU still uses today. I have his original reamer in my possession.
Several years ago, a guy approached Dave Brennan, Precision Shooting about the problems with 300WM's and the above issues. Brennan had several LR gunsmiths respond in a letter type forum. Many were unaware of the problem.
Before the 300WM, the US Military shot the 30-338 Winchester Magnum cartridge wildcat, headspacing the cartridge both on the belt and datum line. Excellent accuracy was obtained with this method.
The Accuracy gunsmiths who responded to Brennan's query determined several things based on the information provided.
1. The shooter picks a lot of brass. The gunsmith averages 10 pieces of the brass for datum line length. The smith lathes off the belt on a couple of pieces of the brass that are the absolute longest in the lot. Uses them for headspace gages.
2. The gunsmith will cut the chamber in to headspace on the now beltless brass. Generally the reamer will not go in far enough to start the belt cut.
3. The belt cut is then done on a lathe to a chosen headspace based on the lot of brass' general average headspace on that lot of brass.
4. Given this method, using new factory loads, the pressure could be expected to be slightly higher but not to a dangerous level.
5. Using this method, reduced the stretch of the 300WM brass, increasing reload numbers, and consistency in accuracy.
6. Using the shorter once fired brass, reduced the maximum load usable in the 300WM but maintained the velocities with load developement. The number of loadings in this type set-up can run from 15-20 reloads.
The downside is generally having to have a reloading die made for the rifle that meets the dimensions of the shorter fired cartridge.
Starting with a factory rifle, obtaining a chamber casting, and using a case gage to determine the maximum chamber dimensions, will give you an idea about the approximate number of reloads you may expect from your rifle with your brass.
With a case gage, the casting, and your chosen lot of brass, you can determine the stretch of a fired case and also decide if your reloading dies will over work the case when resizing.
Lots of trouble, but worth the effort for max accuracy with a 300WM.