The way it works is that when you spawn an HMG, it spawns a Player vehicle which is just the stand, and then it spawns an M2 or DSHK player vehicle, and mounts it to the stand. In that sense, you could mount the stand to a physics vehicle and mount to it easily, but when the stand's mount is occupied by the gun, it wont allow you to switch to that position because it thinks it's already occupied by a "player", in this instance the mounted gun player vehicle. I can work around it by packaging serverCmdNextSeat to detect when there's an available turret to mount to and accommodate for that which is what I'm doing now