If you are within the 5" ZOC no you cant. Your men are mustering up into assault formations and getting ready to attack. By the rules those outside of the ZOC can return fire with modifiers.
A smart defender will have troops nearby so he can feed men into the BUA to replace any you have killed. Doing that will make any trivial shooting casualties you cause make shooting them out as timely as an assault itself.
Musketry is going to be ineffectual unless your game has no time limit. Artillery is the way to go and more historical.
Though it's not written in the rule I'm (almost
) inclined to believe Barrys intention was to not allow musketry into BUAs.
Joe