Reason you can't right click to add silencer's is that the two weapons are different completly, and in different weapon load outs.
We didn't have weapon loadouts in Beta 1, but instead whatever the server host wanted. Now we can pick between Automatics, Explosives, Shotguns, Random, Etc...
80% of the noobs will spend less than an hour in game, not that important, it's only a color.
When adding the silencers, the developers got the ZMG and the D5K mixed up. There was never supposed to be a silencer on the ZMG. Usually when people bring up wanting a silencer on the ZMG, there opinions go right out the window.