@ Bod Man: vehicle mounted weapons are common, but they are ring mounted and pintle mounted.
For visual reference,
Ring mount in a Humvee roof:
https://upload.wikimedia.org/w...9e/M2_-_24th_MEU.jpg
Pintle mount on a boat:
http://cranetechnologiesinc.co...4H60SmallPintle1.jpg
A pintle mount is just a post a with swivel that lets you turn, raise and lower the attached gun, and it could be easily fabricated by a character like Jack. In Fury Road, the warboy behind the .50 cal machinegun standing on the back of the dune buggy escorting Furiosa who said "Why??" when they changed direction was using a pintle mount. Yes, I saw Fury Road again last night >:D
A ring mount is a short pintle mount attached to a rotating ring assembly in the roof of the vehicle that the gunner turns with by leaning with his body weight. It's pretty easy. In a post nuclear setting, you can put a ring mount anywhere the gunner would have enough room underneath the mount to stand or crouch so he could shift his body around. The roof is the obvious choice, but a custom built rig with a deep trunk might allow him to crouch in a ring mount. The big advantage of a ring mount is that the gunner can keep most of his body under cover in the vehicle and he can his gun to bear in a wider arc using much less space to move his body around. In a vehicle battle, the pintle mount gunner will be far less protected and the ring mount gunner will have a huge advantage.
EDIT: What is not common are remotely operated weapons either electronic or mechanical, like most of the vehicle weapons in the Car Wars game or airplane mounted weapons in real life. Example: on the M1A1 tank, the .50 cal machinegun could be fired from the inside by a mechanical solenoid connection when the vehicle was totally sealed. These still exist on prewar military vehicles, but to have a custom built rig with this kind of gear would require an engineer and crew who knew exactly what they were doing. That is a story in itself.
This message was last edited by the GM at 18:23, Sat 23 May 2015.