Directly responding to Newtown, the President should issue a Homeland Security Presidential Directive (see Wikipedia) banning the sale, carry, or use of assault weapons - ALL assault weapons, "grandfathering" none. All assault weapons would become felony-possession weapons, pending further legislation. Sales would be banned retroactive to the date of the Newtown massacre--voiding and illegalizing all of the "buy before the ban" sales since that date. Compensated weapon turn-ins at local police stations should be allowed for 90 days, following which direct seizures and arrests would follow for defiant owners. Pre-Newtown weapons would be "confined to quarters"-carrying them outside the owner's home, even in a locked case, would be felonized.
Events such as the Newtown massacre constitute a NATIONAL SECURITY EMERGENCY. Mortal threats to children in schools by domestic terrorists such as the Newtown murderer are very high at the high end of the spectrum of national security concerns--just a little bit short of nuclear attacks on the homeland. So the President has the right to pre-emptive Executive action without immediate Congressional or judicial mediation, in defense of national security and the nation's children.
Now is the time for the President to use the full force of Executive authority--directly in the face of the NRA, the compromised Congress, and the Supreme Court (emasculated by the obsolete Second Amendment). Presidential Executive Orders are the proper response under the circumstances. Let the NRA and their stooges in Congress scream and sue--meanwhile the massacre weapons will be pulled out of circulation across America.
Later, we can work on repealing the obsolete, disastrous Second Amendment: "Since we don't need no militia no more, let it be clear that no citizen has an innate right to keep or bear arms." Firearm ownership and use should be a revocable privilege administered by the States, handled exactly like motor vehicle licensing and registration--with mandatory pre-license testing, periodic license renewals, annual fees, inspections, and insurance requirements.