I still find the MIU Interface slightly confusing.

So, one can fire the MIU weapon as a free action, no matter what other actions they are undertaking. It says "one additional" weapon, implying that it can be fired in addition to any other weapons held. However, in the errata, it says that they are still limited to one attack action a turn.

How exactly does this work?

a) The character can fire a held weapon and the MIU, but they just have to take the same attack action with each

b) The MIU weapon can only be fired in addition to other weapons as part of a two-weapon attack, requiring the talents, that doesn't suffer off-hand penalties

c) The MIU weapon can be used by itself, if the character is running, working on a door etc., but you can only take one attack action per turn, so without using two-weapon fighting, it can't be fired as well as another gun.

You can make one free attack per turn no matter what else you do, as long as whatever else you do isn't another attack action. If it is, you need two weapon fighting. So you could make a tech-use test and fire a gun at an attacker in the same round. You could Run and fire a gun. Etc. Nifty when a Psykers gets one. Psyker power + Fire a gun in the same round.

Surprised you didn't say anything about the "superior" ballistic mechadendrite that requires a half action to use.