Can someone point me in the right direction to find data on loading 2 1/2" shells? Your own recipes are welcomed as well.
I've been loading 12 gauge 2 1/2" shells for a couple years and quite satisfied with the results.
Basically, I used the same recipe for my 12 gauge 2 3/4" loads with 1/2 and 3/4 ounces of lead shot using Alliant Extra Light powder. I've been loading those for 6 years and had them tested by Tom Armbrust back in 2015.
I use 12 gauge 2 1/2" RST hulls (Cheddites I believe), Cheddite primers and CB6100-12 (neon green wads). Probably not necessary but I trim with a scissors about 1/16" off the ends of the petals of those CB wads.
For 1/2 ounce: I started with 14 grains of XL powder with a Circle Fly 0.135" thick card wad in the bottom of the CB wad. Those tested out at an average 3100 psi pressure and 1187 fps. I bumped the powder charge up to 15.5 grains of XL, same wad etc. and they tested out at 4600 psi and 1373 fps. It burns a little dirty but does well for me.
For 3/4 ounce: same as above: 15.5 grains XL and the 0.135" thick wad is not needed. I couldn't find my test data but IIRC, the psi is up a little and the speed just a little lower.
I pretty much just shoot 12 gauge 1/2 ounce loads in all my 12 gauge guns for my sporting clays and skeet shooting. Occasionally, 3/4 ounce for longer shots.
Ballistically, those 1/2 ounce 12 gauge loads work quite well with the speed you'll get, around 1325 fps.
As for the mechanics of loading them, I use my MEC Sizemaster and add a 1/4" thick spacer at the pre-crimp and final crimp stations as pictured below.
They load nice and shoot nice. And by the way, I shoot those 12 gauge 2 1/2" loads out of my H&H back action hammer gun with steel barrels Nitro Proofed for 1 1/8 ounce.
Please let me know if you have any questions.