I have been thinking of trying a shell casing pen too. I have 22-250, 220 Swift, 243, 260, 270 and 30-06 cases laying around. I am thinking the 270 is probably going to be the test run case. Put a few of them in the tumbler last night, should be nice and shiny tonight when I take them out. The 7mm tube will fit easily inside the neck. With a little work I think I can put a bell on the end of the neck to match the diameter of the nib, if not I will probably just trim the neck off back to where the diameter of the shoulder matches the nib.
So many things to try, so little time. LOL this could get interesting.