There appears to be a bug with the T-28 machine gun controls.
When taking control of the gunner position, both the T-28 and T-28E have 3 machine guns installed. However, on the standard T-28 only one machine gun becomes available and usable, while on the T-28E all three machine guns can be controlled and fired.
This seems inconsistent because both vehicles should allow access to all available machine guns when controlling the gunner position.
Please investigate and fix this issue so the standard T-28 can also use all three machine guns when the player takes control of the gunner, just like the T-28E.