Note: have not tested this with other players (only AI) but I assume it works the same way

Despite the fact that entering a static Mortar, HMG, GMG, etc only requires the gunner to place his hands on the weapon, the gunner gains considerable armor while in the weapon.

This is most notable while firing on them with Thermobaric weapons (such as AP rockets from jets & launchers) and observing falling damage.
Weapons such as the AP rockets on jets (which do damage via large area of effect) can destroy huge swaths of men, but static weapon gunners remain unscathed.
Additionally, a static weapon can fall from great heights, but the gunner suffers no damage (as far as I'm aware).