technically since this is a railgun, a 'silencer' would consist not of containing the shot's expanding explosive gas wave, but of underpowering and sound baffling the charge/discharge accelerators. The shot needs to be subsonic in order to be 'silent' but this is simple to achieve with a railgun by simply dialing down the power output. However typically achieving sound baffling of the accelerators would typically result in a bulkier, not longer, pistol.

This extended barrel does have merit however as a means to achieve better ballistic performance despite lowered acceleration; more linear accelerators in a longer line means better stability and a flatter trajectory, even at subsonic speeds. It also means you can lower the power even more per acceleration coil and not have to baffle the gun potentially at all.