First off you can still get the small reloading manuals from Alliant and Hodgdons just by calling them. Go to their web site for the phone numbers. I find them much easier to use than their web sites. All my loads are under 8000psi and normally under 7000. With that said 2 3/4" shells are ok to use in 2 1/2" chambers. If you still want to shoot 2 1/2" shells here's a easy trimmer to make :
https://youtu.be/JrULU2fslRE Then all you'll need is a roll crimper from BPI or Precision Reloading [ the PRs are better ]. Find a used Mec 600 and buy a "short kit", also from BPI or PR. If you roll crimp 2 1/2 shells all you've done is cut off a 1/4" of material from the end and a 2 1/2 shell roll crimped uses a 1/4" less material. A short kit won't be necessary in this case. The finished shell fold crimp or roll crimp is the same length. If you're going to fold crimp the 2 1/2 shell then a shorter plastic wad or fiber wads will be necessary. With Clay Busters [ and I've called and ask them about doing this ] just use a wad intended for a larger payload - say one for 1 1/4 or 1 3/8 oz and only put the 7/8 or 1oz load in it. The cushion area is shorter. The fingers may stick up over the shot but will fold over with the crimp. I don't know how well the gas seal was in my Parker hammer gun with .745 bores but I still shot occasionally fiber wads successfully. KY jon had good advice. Here is a load I saved to my email - it may help:
Paul Harm;
Here is a load that I have used. I think you will like it. It has been tested as a 1 oz. load @ 1220 FPS 9,400 PSI. It does cycle my 1100 perfectly. Using the CB0100-16 wad and 1 oz. of shot, that is a nice load also.
Remington 16 ga 2 pc plastic hull
Winchester 209 primer
20.0 grains Unique
CB0078-16
7/8 oz. lead shot.
I wouldnt hesitate to use 18.0 or 19.0 grains of Unique, if a guy wanted to reduce velocity.
There are many tested loads using 17.0 to 21.0 grains of Unique, with various wads and both 7/8 oz. and 1 oz. payloads, just not the particular wad you are using. Many loads use the WAA16 which is the basis for the CB0100-16 and the CB0078-16. There isnt one I have tried that wasnt a nice shooting load.
Maybe bump it down for less pressure. Good luck - Paul