Well, they do things when you design the BA...
For that reason it's a bit shaky to give them any extra rules on the tabletop - they are usually used when they already provide a benefit.
But if you really want to complicate things:
Fixed mounts are more stable, but harder to use when moving. Body-mounted weapons get a +1TN at short range if the attacker moved, but a -1TN at long range if it didn't.
Turrets are the other way around. Turret-mounted weapons get a +1TN at long range if the attacker moved, but a -1TN at short range if it didn't.