If your MEC Sizemaster is set up to load 2 3/4 inch hulls, you can easily make a 1/4 inch thick spacer to use at the last two stations for your 2 1/2 inch hulls.
I've done that to both of my MEC Sizemaster's, 12 and 20, and it works fine.
(You may notice I set up my MEC for loading in that box with sides all the way around to catch those little BB's (shot) that seem to get away.)
