Your miroku over an under only fires one barrel at a timei thought that once you loaded it you should be able to pull the trigger an fire which ever barrel you have selected first then fire the second?

In a Miroku over-and-under shotgun, you typically have a selector switch that allows you to choose which barrel to fire first. Once you've loaded the gun, pulling the trigger will only fire the selected barrel. After firing the first barrel, you can then pull the trigger again to fire the second barrel. If it's only firing one barrel at a time, it may be functioning correctly, as that is how these shotguns are designed to operate.