Hi and welcome to the wonderful world of shotshell reloading! It can be a lot of fun and a lot of trial and error. Couple of suggestions.
First, BPI has some nice products, but stay away from their manuals. The data they published can be very sketchy. Stick to Lymans 5th or the powder manufactures load data. They have been tested. BPI is done using a computer program IIRC.
Second, if you want some great info on reloading here's the place to go to:
https://www.shotgunworld.com/forums/shotshell-reloading.13/Tons of great info and folks who will help you out.
Third, you can shoot 2 3/4" in a 2 1/2" chamber. Sherman Bell has done extensive testing in his articles, "Finding Out For Myself" published in the DG Journal. Do a search on this forum or Google it.
Good luck, have fun and take your time!
Best Wishes on your success
Greg
