I contend that a heck of a lot of police will refuse to carry out any such orders and the same goes for military personnel (but they can't be used anyway - right??). There would be some who would carry out illegal orders such as this but once large pockets of resistance sprung up and shooting started, they may dissolve any sort of disarming ideas.
Any way you look at it, it would be a messy affair and you can bet it would get bloody. I seriously doubt we are going to see confiscation starting up but nothing would surprise me. I think they know that trying something like this would be a bad mistake.