...is doing the same thing over and over and expecting different results.
Time and again, gun registration schemes lead to massive non-compliance, are ignored by criminals, and eventually turn into a handy checklist for confiscating authorities to come by and pick up the guns.
Every time a registration scheme is proposed, gun owners point this out and get derided as tin-foil-beanie-wearing yayhoos, and every time it keeps happening just like it did before.
It is an easily demonstrable fact that gun registration does nothing... indeed can do nothing... to prevent crimes: I will write my name and address and the serial number of a gun on this sheet of paper and hand it to you. Then I will leave the room. Using that piece of paper and nothing but that piece of paper, stop me from shooting something with the gun. I'll bet you twenty bucks you can't.
The only way you can use that sheet of paper to stop me from shooting something is to hand it to another person with a gun and tell them to come get mine.
And maybe that's what you wanted all along, right?