I was certified as one of the first MCS officers in the state and taught MCS at our academy. Did a lot of truck inspections in 27 yrs. Unless they've changed the MCS law in the past 6 mo there is no MCS law prohibiting carrying firearms. States adopted the FMCS laws and some added additional restrictions so while there is no fed law there could be a state law under that state's MCS laws. In addition, any state law regulating firearms would apply and MCS would not make the driver exempt from those state laws.