The solution is to get more guns and better training for the people who legitimately can own guns. The felons and truly mentally ill should not. Every county should develop a large posse from as many combat arms military veterans as are willing. That would be huge numbers.
This kind of thinking betrays the reason for a Constitutional form of government. Posse groups have no business in issues of state interests. They are privateers with private interests. When disputes between people arise in a Constitutional democracy, we tacitly agree to allow the state to conduct investigations where citizens cooperate with the state to establish the facts and evidence. Then private parties are allowed to argue those facts in a court of law to resolve disputes. The law is upheld by the court as the sole interest of the judiciary, which allows for state interests of impartiality to the parties to exist. The state is only interested in the law being applied to all parties. Posse action, being private interested only, naturally is biased as there is no governance to their actions. Therefore, putting posses together with armed citizens is nothing more than jungle law, where justice is based on the interests of the most well armed. Armed citizens are the worst form of justice as investigation and evidence establishment are denied, meaning no justice results from any action of the posse. Posses don't gather evidence with respect to the law. The squeeze triggers and kill in the name of justice. There is simply no due process actions by vigilantism. None.
Combat veterans make lousy investigators. They know how to control space, but they are not trained in forensics or investigation. Bad, idea. Really bad idea.