Yeah, its basically a bug on 1.6, or an exploit, so they fixed it for source, as good as it was, they're trying to be more realistic, for example you have to wait for the silencer to screw on completely and his arm to go back to the normal position before you can switch weapon and the silencer stays on, whereas on 1.6 you just begin to put it on, quick switch to pistol then back to gun and the silencer is on.