out there already. Unless there was to be a forceful outlawing of the possession of weapons and a collection of them by law enforcement, the weapons already exist in the general population. Even if all new sales were prohibited, there'd still be PLENTY of guns available for folks who want them to get them. The same way that there's plenty of drugs available for those who want them.
In fact, a time is coming in the very near future when regulating the sale of guns is going to be irrelevent anyway, as you'll be able to easily manufacture one in your own home.