I have posted an answer on this topic more than, once, but it's not in the President's power to enact on his own --
A year of compulsory national service, thus taking the "well-regulated militia" phrase of the 2A seriously. The year could be accomplished within 5 years of HS graduation (or by age 25 if not a HS graduate), consisting of basic military training, specialty weapons/tactics training for those capable and willing, and service in a hard-to-fill billet for the remainder of the time, perhaps just KP duty in support of the basic/specialty training. Successful completion of the weapons training would permit the citizen to possess a standard weapon issued to him to be maintained in his residence for use in times of national need. The "year" could alternately be completed by a 4-year program similar to ROTC with a reserve officer's commission at college graduation. For those opposed to military training, an alternate one year (or four 3-month segments) of service, similar to local community service programs but on a national scale -- things like National Forest/National Park grunt work, service as an orderly in a VA hospital, or something similar.
This would not only have the benefit of added manpower at low cost, but also the benefit of weeding out those who should never have access to powerful weapons.