This important question is of course basic: how did it actually work?
Sadly, I cannot explain either.
Of course, the armored knight in the piece of period artwork attached was obviously not able to fire his combined mace and gun single-handed. How he could have possibly done this on horseback is left to speculation. Even if he did not have the reins in the other hand, this would have had to be much longer than the one that held the club, in order to reach the touch hole!
This leads to the basic question about the practical use of almost all Medieval and Renaissance combined weapons. I am afraid that most of them were mere fantasy put into reality by arts and crafts, but totally useless and inefficient. The Early Renaissance taste, based on a brand-new self-consciousness, Renaissance man was keen to make virtually any idea become reality - totally regardless of its use. In art history, this phenomenon is called
ludus globi - the unlimited philosophic play of the mind which maked the world a toy. Any device was realized just to prove that it could be done, by the human mind.
Please also check an absolutely stunning and wonderful piece in my collection, similar ones - exactly as useless as this but ordered for Henry VIII's armory in about 1540 - are preserved in the Tower of London, the Royal Armouries Leeds and the Met:
http://www.vikingsword.com/vb/showth...t=meyrick+mace
Best,
Michael