Good point but how to implement an effective program and whether such a program would have made a difference in this case are valid points.
I carried a weapon for all of my waking hours except when taking a shower or going to the gym during a total of 18 months in Afghanistan. Although, I'm a physician, I did learn to really enjoy becoming proficient with the M4 rifle and M9 pistol, and I understand Mrs. Lanza's interest in target shooting. It was In hindsight clearly an awful mistake to try reach our to her troubled son by teaching him about weapons as well, but I can understand what she was trying to do. People who enjoy target shooting are not horrible people. She clearly underestimated the degree of evil within her son, which is likely a common trait within other mothers of monsters.
So, how could an "assault rifle" ban be implemented? Which weapons fall in the category and which do not could certainly become an issue of serious litigation. Many jobs in the arms manufacturing area could be at jeopardy. There are much more dangerous rifles and weapons than the AR15 used in this case. But what about the AR7 survival rifle that shoots a 22 caliber, only takes 7 rounds in the magazine and is designed to be disassembled and put into the water-tight buttstock...it is really not a dangerous weapon that would be misused like this case. Some pistols are just as dangerous as assault rifles. Nidal Hassan, the jerk who was the year behind me in med school, used such a pistol, which fired high-velocity rifle ammunition in his dastardly work at Fort Hood.
So, even if you come up with a fair definition and ban the production and sale of assault rifles, there will still be millions of Americans with such weapons in their homes. Many of my fellow military members in Afghanistan own personal weapons, and many had several thousand dollars invested in them. Voluntary buy-back programs at $200 or less in general, collect weapons worth less than $200. Outlawing the ownership of such weapons would never fly without reimbursement. However,banning new production and sale of the most dangerous weapons (rifles and pistols) and limiting magazine size to say 7 rounds and allowing no one to carry more than two magazines might fly.
Regrettably, this wouldn't have helped in this case, as he killed his mother and stole her weapons. (BTW I own no personal firearms)