Oh and my 2 nuyen, I would agree with modifying the mount as being part of the armorer check to swap out weapons. Maybe add +1 to the difficulty for some of the non-standard stuff (say assault, battle, and sniper rifles) and a +2 for the fairly weird stuff (like a pistol or SMG).
Maybe for a large, standardized mount (like military vehicles often have) it would take a lot of work to mount, say, a heavy pistol or an SMG, because they're not designed for guns that small. With the popularity of smaller vehicles and drones mounting weapons, there is likely very simple mounting hardware available in 2070 that can accommodate smaller arms without difficulty. Clamps and a trigger-pulling mechanism could allow a light mount to easily be used with almost any light firearm. Look at the rigs used to hold guns for testing, then put that in a turret or on a servo. It would require adjusting for accuracy and such, but it wouldn't be significantly more complicated than mounting a heavier weapon in a heavy mount.
Making the heavy mount universal would be more difficult, because the clamps wouldn't likely be enough to hold the gun in place while firing on full auto, so it would need lots of hols, bolts and pins to fit all of the different specs of the different major manufacturers. Unless heavy weapon mounts have been standardized into a few set configurations by the Corporate Council, like USB connectors today.
