Gauge, choke, and load size all have to do with the pattern you will get. Your not going to get an 1-1/8oz 28ga load to start. 1oz 12ga will have less fliers then the 20ga. Also the shot collume will be longer with the 12ga. the longer the shot cup the more contact by the shot traveling out of the barrel.
I have chukar hunted in the middle fork of the salmon in ID. I can tell you this the 28ga legacy would be an awesome gun for the job. Both the 20ga and 28ga will kill them stone dead. Shells is an issue or I should say could be an issue for the 28ga. 20ga I have never been to a WalMart, gunshop, sporting goods shop that didn't have 20ga. Not the same for 28ga. With the 28ga I would shoot a tighter choke like light Mod over dogs, in the 20ga I would shoot skeet or IC.
