Jeff:

3/4 inch diameter dowel. 5.5 inches long.

-Victor

That's a lot quicker and easier once you find the correct length. I may make one like that for 12ga but put a screw in the end of the dowel so the length of the shell could be easily changed for roll crimps or even 2 1/2" shells. Thanks for posting.

Ive shown this little jig I made years ago, several times. I use it for 12-28. The trimmed end is every bit as square ans smooth as the NPEs that I'm cutting down. It's fast too. I just did 300 new 28ga hulls while watching Yellowstone last night.
If the pictures aren't enough for you to build your own, then buy one, because it couldn't be any simpler, and with all respect for everyone's home built trimmer, they don't compare. Yes, I'm bragging, but I'm a retired engineer and still subscribe to keeping it simple, but making it the best quality I can,