In California around 5 to 10% of crimes are committed with banned guns, and for gang and drug crimes it can be 30 to 50%.
Cite your sources! Also, what type of "banned" weapons are you talking about? In Cal, a banned weapon is anything with a magazine capacity of more than 9 or 10 rounds (IIRC). So a Glock with a high capacity magazine is "banned", while it is perfectly legal in NV. There is a big difference between a semi-auto pistol with a "high capacity" magazine (banned in Cal) and a fully automatic weapon.
The idea that "seizing" all of the class 3 weapons on the registry will stop crime with these weapons is absurd. People own these weapons LEGALLY! This means that they have gone through major hoops and paid lots of money to buy something, but more importantly they are law abiding citizens. If you want a fully automatic firearm to commit a crime, there are easier and cheaper ways to go about obtaining one without buying something from the pre-1986 ban. Illegal full auto weapons don't come out of the collections of law abiding citizens. They are smuggled into the country for criminals, by criminals.
Why should I, as a law abiding citizen, have to surrender my property to the government because someone else commits a crime with a similar item? What is that going to accomplish? Should we ban and seize all cars because a few people intentionally commit vehicular homicide (running people down with a car)?
People need to be educated about firearms... about the differences between semi-automatic and fully automatic weapons. About what the true meaning of "machine gun" is. And why the term "assault rifle" is essentially meaningless because there is no definition as to what one is, or more importantly why an assault rifle is, at its core function, any different than any other rifle.
Criminals can and will always be able to get ahold of guns. Why does the argument follow that it is OK to deprive law abiding citizens of their firearms?