Hello all, I am looking for some advice/help. I have been reloading for about 12 years now but I have run into an issue and need help. I just bought a used 30-378 with a 338 neck. It came with fifty new 378 belted cases that have not been necked down. It also came with some loaded cases and fired brass. All of the loaded cases and fired brass have no belt. I can see the trim marks so this 378 brass has had the belt removed. The Full Length sizing die has “No Belt” stamped on the side as well. So, my issue is two fold. How do I trim off the belt and size the neck on the new brass? I ran 3 pieces up the Die but ruined all three by “mushrooming” the bottom of the neck. I did some internet searching and came across a thread mentioning that I need to neck turn some of the brass off the 378 brass first so that there was not so much brass to be sized and this would keep the bottom from “mushrooming”. Is this the correct method? Lastly, I am confused on the 338 neck. Does this mean that I cant buy 30-378 brass since the neck is .337 or do I go ahead and load 30-378 rounds, shoot them and the neck will then form to the .338? I have rifles where I neck turn down but none that “turn up”. My dies are “No Belt 30-378 with .338 neck”. Looking for the best way to handle all of the above. The used and loaded brass will only last so long so I would like to be able to make my own. Thank you in advance for any and all help. David