They could pass a raft of legislation mandating chronic costs and hassles.  Then any infractions could be used as justification for draconian punishments.  This is how I would 'get the guns' if I were designing a strategy.

The above strategy probably would get the guns out of the hands of most typical law abiding people, but not out of the hands of criminals who create problems for ordinary folks.  That is actually fine with TPTB.  Violent criminals are a very useful tool that TPTB can use for system control purposes and to justify 'official protection' in the form of paramilitaries.  Both the violent criminal classes and the state sponsored paramilitaries are more effective in certain kinds of operations, and cost effective, if the general citizenry are dis-armed.