Gauge has so very little to do with killing power and is so misunderstood it is crazy!
I have some factory 12 gauge 2 and 2 1/2" 6's and 7's that kill pheasants just fine with a 28 gram load of shot.
I also have some old factory 2 3/4" 16 gauge ammo that is loaded heavy with a 1 1/4 oz of lead 6's and 7 1/2's at 1,200 FPS and some 20 gauge factory that us loaded with 1 1/8 load of 4's the same speed
My 28's are loaded at 7/8oz of lead at over 1,300FPS and some 20's that run closer to 1,400FPS.
My 12 gauge vintage gun runs about 1,175-1,200 FPS with a 1oz load of shot
and my Lafaucheaux 16 shoots a 2 1/2" shell with 7/8 oz of shot at about 1,150 FPS.
all of these shells have killed pheasants and partridge cleanly wild pheasants as well as released birds.
Chokes? Meh- my partner did wonderful this year with a 20 gauge full choked A5, it FITS him better than any shotgun he has ever shot including a 725 browning with screw in's, a CZ O/U, and a fine little Belgian Guild SxS in 20 gauge.
My main gun is 12 gauge CYL/CYL but every now and then I shoot a 20 with screw in chokes or a 28 with fixed IC/MOD barrels.
A fixed choke IC/MOD gun will do just fine as long as it fits regardless of gauge.
Has fit been mentioned yet?
Cat