Let me ask you a question.
Say we do ban all semi-autos.
How does this help reduce guns obtained through the black market?
Because a ban would just move the cheaper semi-automatic into the black market, and only the criminal type will purchase them.
What black market?
Most guns used in crimes are obtained illegally through an FFL.
frontline: hot guns: "How Criminals Get Guns" | PBS
So you find the illegal guns, pull them off the street but they are constantly replenished via FFLs. If they were banned, eventually the source would dry up.
All of these guns were legal at some point. Making their sale illegal would cut off the source. Then yes the problem would to round up all the illegal guns.
At least as they are found and removed they would no longer be replenished.