Possibly the easiest and most available factory load is the Winchester AA low-noise, low-recoil load. Even though it's a 2 3/4" shell, the 6,200 psi is low enough for use in 2 1/2" chambers. It would be suitable for Trap and smaller birds, and runs about $8 per box in most of the country.

Regards
Ken

I prefer wood to plastic, leather to nylon, waxed cotton to Gore-Tex, and split bamboo to graphite.