The gun and the dies from RCBS are both marked 338=300 Win. L.N. and the long neck cartridges with the the long neck that I loaded fits in the chamber properly. I'm sure Steve Billeb being a Master ACGG Gunsmith would know what he was doing when he built the rifle and marked the caliber ... and bought custom RCBS dies to go with the rifle. There is "old school" history about the 338-300 Win Long Neck being used in Long Range competition.

How to Hunt Wisconsin Whitetail Deer with a Cannon