[John Farrell, Charter Member #33]

I searched my old reloading material going back to the 60s and don't find any 20 gauge 2.5" reloads. As I understand the difference, the light 2 3/4" loads recipes with lo pressure might be suitable for use in the shorter shells. The difference between the 2 1/2" shells and the 2 3/4" shells is that the ammo makers, when going to a folded crimp from the historic rolled crimp, simply added a 1/4" to the hull length because the old hull would not fold crimp due to a lack of material in the hulls they had to work with.

There are some smarter folks on this site than I am and others may have some really ancient reloading material that includes data for you. Good luck.