There's some kind of locality issue when there are multiple players in the vehicle. It seems like the railgun functionality requires the vehicle to be local, but locality isn't changing or the change isn't being detected by the railgun functionality.

edit: or the functionality requires the vehicle to be local to the gunner, which poses a problem when there is a full crew with commander/driver, since it is not local to the gunner in that case

Yeah. Did some tests tonight. It appears that the gun stops working as soon as there's another player in the vehicle. If there's only one player+ai, first shot is wasted but then it works without problems

Full repro (DS and multiple players required to be certain, but may also happen in P2P MP):

• place a T-100X and 2+ playable units
• host on a server
• have two separate players play as the playable units
• one gets in the commander's seat, one gets in the gunner's seat
• while there are multiple players in the tank, the main gun does not work. A single round is subtracted from the ammo count, but the charge mechanic does not work and the gun doesn't actually fire.
• repeat, but only one player gets in the tank, as the gunner
• now the gun works

You can play around with different arrangements of driver/gunner/commander to find the exact conditions.