Do homemade guns circumvent any firearms laws?
Sure. Most of them. The 9th Circuit even ruled that if you make your own machine-gun, the Federal laws do not apply because it is not interstate commerce. Unfortunately, I believe that BATF hard lobbied most states to copy the federal legislation. That was probably not legal, but when did the BATF worry overmuch about legality?
I might add that circumventing laws is not illegal.
No, as long as you keep shotgun barrels to proper length, and stay away from full auto's. You could build and register full autos for a long time but that was halted some time in the 80's(I think that was the time period).
Depends on what you mean by circumvent. If you mean are they legal, then yes, in general. You can't make a machine gun, but you could copy a 1911 or an AR for example. Home made silencers are still regulated like store bought, but you can apply for a permit.