Three things:

As mentioned the BP shells are as promised as Steiner said, shoot great

RST uses Perfect Pattern in their 2.5" 12g shells but I don't know the load data

Fiocchi 1170fps 1 oz are digestible in about any 12g Parker regardless of chamber length, but that's just my opinion as I've shot about 10 flats of them. And they are available.

I really like B&P shells in 12 and 20g. Soft shooting and turn targets into diesel smoke. I shot some of the factory B&P 7/8 oz 1160 FPS today. They break targets and feel like you're shooting a 28g gun. Just sayin'.