300 win mag problem

[Jimmy's]

Yet millions are using and sizing belted cases everyday.

[ar15barrels]

I am not quite sure what you are saying here.
Are you saying to file down the bulge in front of the belt and make sure that that is what is causing the issue?

Yes.
It is either the case bulge forward of the belt OR the actual shoulder length that causes a cartridge to not chamber freely.
By filing down the bulge on one case which fit tight before you filed it, you would learn if it was the bulge or the shoulder causing the tight fit.

If it is the shoulder, then you do not need the willis die.
You just need to properly adjust your existing full length die.
If filing the case reveals that the case fits fine, then you need a willis die or some other die which will better size (small base) your cases to fit your rifle's chamber.