from a good friend who decided after a lifetime of gun ownership to sell his assault weapons:

As far as bans/controls going forward, if the line is going to be meaningful, it has to be drawn at semiautomatic weapons with detachable magazines.  This is lenient enough to allow bolt-action rifles, which need the bolt to be worked between each shot and are the overwhelming practical preference for hunters and marksmen alike, while being strict enough to exclude what an almost comically ill-informed media describes as “military-grade” weapons.  The common denominator between the handgun that a gang member uses on the street and the AR-15 that a schizophrenic uses to kill moviegoers is not that they were designed for military use in combat, but rather that they both a) fire one shot per trigger pull without reloading between shots, and b) accept rounds from a detachable magazine of varying size that can be switched out for a fresh one in a matter of a couple seconds.  It’s this technology that escalates a shooting from one or two dead to twenty or thirty dead— you can get more rounds in the air faster, and there’s almost no reload time in which you’re vulnerable.  

That’s why I’m already frustrated that people are talking about renewing the Assault Weapons Ban of 1994.  It was a meaningless piece of legislation, because rather than addressing what really makes these weapons so potent, it defined an assault weapon as any firearm that has more than two of an enumerated list of features— things like a pistol grip, a bayonet lug, a collapsible buttstock, a bayonet lug, a detachable magazine (thankfully), or a grenade launcher attachment.  So what happened is that manufacturers continued making the same firearms, but they would simply hack off the bayonet lug or pin the buttstock in the extended position.  Using these tricks, the rifle that Lanza used would have been completely legal during the ban.  I bought one myself during the ban.