Isn't there a "homemade gun" scene that operates on the idea that a weapon whose resources are collected in a single state, and that is assembled is that state, is not subject to federal firearm restrictions? (Since many if not most federal regulations rely in the commerce clause)
The point is, the federal government does not have a delineated power to regulate firearms or silencers. When they outlaw them, they say they're regulating "interstate commerce", which is a power they are granted.
If you manufacture the silencer yourself and don't cross state lines, and the materials you used also didn't cross state lines, the federal govt can't regulate it. (Although the state can.)