Haven't hunted grouse successfully, so can't be much help there, but have done a lot of dove hunting. Back when I was using suppository guns, I tended to use 7 1/2s or 8s cause they were usually cheapest, but there were times when I used 6s just cause those were all I had, and they all worked just fine. Now that I make my own birdshot, and the only size I have the drippers o make is #6s, that is what gets used for absolutely everything that gets birdshot, my next smallest option is #4 buck, and I'm not going to try that for dove. My two smoothbore MLs are a .54 single and a 13ga (.710") SxS, both cylinder bore, if that helps.