The flavour text on the driver magazines give a supply rate to the drivers, however this can be exceeded, and is especially obvious with the Tetsuo HMX (i presume a limit could also be surpassed when using multiple mass drivers on a smaller magazine.)
The test i performed used two HMXes and a 1000kg magazine, which by the stats should take 10 seconds to completely deplete, however with both HMXes firing it takes only a second.