7 1/2's are good at closer range (inside 40yds) and 6's .5's,4's work a little better at the longer ranges, just fine, the cheap shells are ok but I prefer a heavier game load. The modified choke is great all around choke for crows, a tighter choke will make those 20yd shots tough but will work great for those last shots where they are flying straight away from you.
I bought an over/under this summer and decided to use it on some crow hunts before dove season. I would put a 7 1/2 in the top barrel with a modified choke, and a #5 in the bottom barrel with a Terror choke (.680). Crows are fun to hunt, my favorite sound is the "dying crow" and my "buster" motion decoy.
