Are these reloads? If so you have 2 headspaces to worry about with a 300. The first and most obvious is the belt, which is the true headspace for the cartridge. The second is the shoulder just like in most other cartridges. I have run into this myself. I have a Winchester Model 70 that has an extra long space to the shoulder. Setting the resizing die as directed led to early case failure. Get a bump gauge for the 300 and start checking fired brass against your reloaded brass. You will likely be able to solve your issue with an adjustment to your resizing die. Set the resizing die to just bump the shoulder back 0.001-0.003.
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