I believe that if the powers that be order the police to collect the peoples weapons we will see several things happen fast.
1. Many police will simply refuse because they know it is not constitutional or they know their is very good chance they will end up shot.
2. The police that do go out to confiscate guns will end up dead in short order.
3. The rulers of that state will bring in National Guard troops because they will be out of cops and are incapable of either backing down or controlling the situation.
4. Many of the National Guard troops will refuse for the same reasons many of the police did.
5. The remaining National Guard troops will end up overdoing it and they will kill some people who do not have guns, which will set off the rest of the population of the state and cause protests and/or outright uprisings all over the US.
If the state rulers do not back down it will escalate into a shooting war in that state at the least and possibly around the country. In any event it will be interesting and educational.