We get asked this question quite often.  The answer is: Whichever barrel did not fire last time.

Some other manufacturers of derringers have a selector switch which must be moved from one side to another.  
This is not the case with Bond Arms derringers.

Our derringers automatically swap from one barrel to the next every time the hammer is pulled back.
This happens through the life of the firearm, and it switches whether a cartidge is in the chamber or not.

If you need to know which barrel is going to fire, all you have to do is look at the position of the hammer head:

When the hammer head is close to the hammer, the top barrel will be firing.  When the hammer head is further away, the bottom barrel will fire.



If you need the next shot our of your Bond Arms derringer to be the barrel other than what is indicated by the hammer head (let's just say you had .45LC in one chamber and .410 bore in the other), all you have to do is engage the manual safety, gently lower the hammer, and then disengage the safety. 
When you cock the hammer back again, the hammer head will have automatically moved into position to fire the other barrel.