JD has the right idea. WHen I work birds in preserves, 2 3/4" w/6 works just fine. I' ve even started the habit of using my 28g. on these liberated birds. High plains wild roosters are a different bird altogether, though. Not only are they bigger and tougher physically, but conditions usually require a heavier load. The winds of my homestate are notorious for eating shot patterns. We normally go with max dram 2 3/4" shells with 5' s or 6' s, Mod/Full (O/U).

The guys using flushing dogs go so far as to use 3" #4' s for late season birds.

1 1/4 oz high velocity loads of #5 in 12 ga or #6 in a 20 ga for early season with a light mod choking. Late season 1 3/8oz of #4 at the highest safe load speed I can find formulas to load and nickel plated shot with buffering to boot with chokes starting with light mod to Imp. mod.

And this is WITH a GSP that knows his job. But birds here have enough pressure that in the late season shots are long and few hold for a dogs point.