Adding more laws, such as universal background checks and registration, can reduce fraudulent purchases and dissuade straw purchases. It can start to reduce the supply feeding into the black market.
The great majority of black market guns were legally manufactured, the great majority of imported guns were imported legally.
The fact that guns are widely a easily available on the black market demonstrates that many, many gun owners have been careless with who they sell to or how they secure their weapons from theft. The black market for guns is incredibly damning of the current state of gun ownership, which seems to feed an unending supply of guns into criminal hands. Every year vast amounts of guns move from legal ownership into the hands of those who no longer have the legal right to own them. Not all of that is theft. Not by a wide margin. Even then, there are proposals for laws that would address safe storage, both to reduce accidental/unintentional shootings as well to reduce theft, thereby further reducing the black market supply.
That does not get into laws that can be passed which would create temporary holds on the possession of firearms - so called
"Red Flag" holds.
The NRA has fought tooth and nail against those sorts of holds, yet it is just about the only thing that could have prevented the recent shooting in Florida.
There is quite a lot that new laws can do to reduce gun violence or keep guns out of the hands of the wrong people.
ETA: Even that does not even get into laws that could be passed to end the
non-sensible prohibition on electronic records within the ATF's gun trace program. That and a few other laws seem to be based on the "They're coming for your guns" myth.
