I had several 2.5-inch 20 gauge loads pressure tested by Precision Reloading. The two powders I used were Alliant Longshot and Winchester WSF. The one I liked best, because of low pressures and reasonably moderate velocities, was the following:
New primed Cheddite hull, 18 grains Longshot, BPI brush wad (no shot cup), 7/8 oz lead shot, roll crimp.
Test result for this load (6 shot string) was: avg pressure 7128, avg velocity 1222.
The other loads using WSF and one with 19 grains Longshot and various wads gave higher pressures and velocities than I wanted with a vintage gun.
I loaded up a batch of the above recipe except that I reduced the powder to 17 grains. They gave nice even patterns with #8 shot at 20 and 30 yds out of my Army Navy, and will be my load for woodcock and (hopefully a few wild) quail this coming season.
My LC Smith 12 gauge has 2.75-inch chambers so I can use my MEC single stage press to re-load 2.75-inch Remington and Win AA hulls. I have been loading Alliant Extra-Lite 14.8 grains, Claybuster 0175 wads, 3/4 oz shot, fold crimp. I haven't had this load tested since it is listed on Alliant's website (almost exactly). It is very low-recoiling and breaks clays great. If I needed 2.5-inch 12 gauge shells I would use this recipe with a brush wad, and roll crimp them.