I would use the biggest that you can, and still pattern enough to use. I use 5 and have also used anything I had on hand. Have gone to many crow tourneys and have shot them and had them fall from the sky and hit the ground dead. (I thought) to later have them fly away, when every crow counts, you want them dead. So I think they're pretty tough. I like 4's and 6's but 5 fills the bill nice. Have also used #2 steel. Had some luck w/BBB lead also. Won a tourney with 101 crows in a day. We shot a lot of rounds!! Number 5 shot mostly.