Dear Mr. President:
Most recent violent mass-murder acts were committed by those wearing some sort of "body armor." I know of no Second Amendment right to either purchase or have such "going-to-wage-war garments" in ones possession. The elimination of the ability to purchase or own may reduce the delusional mindset from a state of invincible to vulnerable.
I would request that if you cannot get the guns, then by executive order - through the Commerce Department, enforced by the ATF under the Patriot Act, deem all such items evidence of a "potential terrorist attack,'' thus giving each department clearance to proceed as outlined within said act.
Have a background check conducted on every "lawful licensed owner of every gun" retroactively. This not only your right under law but it is also incumbent upon you to do so as our, "Protector and Commander and Chief."
Announce that, both the ATF along with the IRS is conducting their own joint investigations as to all sales of all weapons, of all manufactures and resellers alike. They may expect a visit by this joint task force quite soon.
I would suspect that you may have much less of an opposition to your goal, and all our goals, to save our children.