What do folks think is a good house rule for hidden weapon mounts? I'm thinking thath they take up an extra 2 "slots" of body, and require a simple action to pop open (possibly strung together as a single simple action for all of a vehicle's mounts).

I'd say it would depend on when they were put in. If its a design put in to a 'stock' vehicle, I could see it costing more space. Fixing my Toyota Corolla so a dual mounted MMG slides out the back would be a significant undertaking. Moreso than say building a car that looks like a toyota corolla that has the dual mounted mmg's from the get-go. I'd think you could better use space and build the car around the mounts if that was the plan from the beginning.

I personally think you should be able to just pop a plate onto it when not in use, so your car looks legit. Then quickly instal the weapon.

One of the assumptions that SR4 makes about vehicle weapons is that they're turreted to an extent (as a fixed weapon would require you to turn the vehicle into the target, and there's nothing mentioned anywhere about having to do this). The hardware not only has to include the weapon and the drum of ammo, but all of the actuators required to give the weapon a pretty good range of motion.

I sorta thought that weapon mounts were automatically hidden. Lord knows they're vague enough about it- I figure if there's just one kind of mount with no rules about it whatsoever (other than what size gun and how much ammo it holds), nothing's stopping you from saying it's a hidden, internal mount. Except your rigger hating puppy kicking nazi GM. But then again if you have one of those, why are you playing?