Then allow concerned and responsible citizens to be trained and tested to the standard necessary. If 5% of the population were trained militia that would make a he'll of a difference.
On a sensible note, once you take out the kids and o.a.ps from the population, you're probably down to half what it actually is, so you'd be looking for 10% of what's left. Once you'd weeded out the Walter Mitty Rambo wannabes (read airsoft fans) and the nutjobs, and the people who don't want to, and the people with checkered pasts, and the stupid ones who couldn't pass the training, How many would you be left