Another option might be a mandrel swap. Lee does not make an undersized mandrel for .303brit. However, the standard size mandrel on the .308win is .3055”. The standard size (the only size) for the .303 is .3085”. The undersize mandrels they offer run about .002” undersize the standard. For example, the undersize .308win is .3035”. So, by utilizing the “standard” sized .308win mandrel, I could increase my neck tension by .003”. I’ll give them a call in a few hours here when they open, but this might be an alternative over the factory crimp die option you mentioned above.

One has to keep in mind here that attempting to squeeze precision out of a combination of barrel, chamber and rifle that may not be entirely capable of it...just might be a blind alley. Hopefully this whole thing will be more in focus later in the week.