The rifle was rebored and regrooved for the 338 bullet without changing the headspace or shoulder angle of the original 303 cartridge.The neck was also rebored for a 303 British round that was necked up to 338. My neck chamber measurements from a fired case are .369 outside dimensions. I will be neck sizing to about .366 od with a .338 bullet seated .This leaves .003 for neck expansion (recommended by Redding) I talked to Redding USA tech shop and they tried it and it worked .The gunsmith who rebored the Speed also used this method to make cases that he tested the rifle with after reboring.he used cut rifling for reboring.I have now a 338/303 . caution must be exersized in using different kinds of brass as neck size may vary.The initial .366 or slightly smaller resized neck must be maintained by outside neck trimming if necessary.So far with Winchester cases I have not had this problem