Originally Posted by weagle
The bottom barrel is fired first. It's more in line with the center of the stock and gives a little less muzzle flip for a faster follow up shot.
The bottom barrel is usually the more open choke. theory being on a quick flushing bird the first shot will be closer and the second shot more distant. Incomer's allow more time for the shot and it's usually simple to switch to the top barrel first.
Weagle is correct ---
As far as he goes---
The bottom barrel is usually the more open choke ---if that is what the situation calls for. For driven birds (birds flushed toward you) you will typically have the tighter choke in the bottom. Likewise, for decoying ducks, it wouldn't be unusual to have the tighter choke in the bottom. Likewise, shooting doves, where most of your shots are some form of incoming or crossing shot, it wouldn't be unusual to have the tighter choke in the bottom.
Over the years, I've gravitated to tighter chokes for dove hunting, which is about all the bird hunting I do now. Still that's a situational decision -- I don't have a good dog now to pick up those birds that I just nick and that fly on for 75 yards. Right now I want to kill them dead, or let them go. Point being there really isn't a pat answer.
As far as sporting clays, most shooters do as Weagle does --put in the two chokes they are comfortable with, and leave them there except for the occasional situation that requires something different. In sporting clays, you are going to miss very few birds because of the choke.
Like a lot of things regarding shooting it's easy to over think it.
It is a fact that firing the top barrel first on a regular basis will cause undo wear on the action, especially a Browning type action. Some other type actions not so much. Even that said, some trap shooters will fire the top barrel, but in that case it's not because of the choice of chokes.