What we know about the Orlando massacre is that the shooter used a "223" AR-15 to do most of the damage. He legally bought it by passing a background check. For those who don't know, the AR-15 and all of its variants is essentially the civilian version of the military's M-16, and has standard magazine capacities of 20 and 30 rounds. That's a lot of shots available to someone who wants to kill a lot of people. Larger magazines are also available.
The normal argument for owning one or more of these high capacity military-style weapons is "for self defense". Arguing that there are bad guys out there with similar capability. This is a reasonably sound argument since Americans have bought tens of millions of them. Others say they are just gun enthusiasts who want them as part of their personal collections.
However, because of the widespread killing power of the AR-15 and other weapons like them, I think that a new federal license coupled with a more thorough background check should be required before a purchase can even be attempted. Perhaps, even a mandatory interview by the FBI or other law enforcement agency. Additionally, there should be an enforced waiting period of two weeks or more before a dealer surrenders the weapon to its new owner. Also, the number of high capacity magazines should be limited to only two purchases per year.
These restrictions would not bar qualified individuals from owning these guns, and, as such, would not be a violation of one's 2nd Amendment rights. It would simply put qualifying time and purchase limits on buyers so that those who intend to kill would be stopped or abandon the plan; especially if they have to be interviewed by an FBI agent. Would this program have prevented the Orlando shooting? I don't know. People who want to kill will always find some way to do so. Just look at the Boston Marathon bombings.
Lastly, while high capacity rifles tend to show up in mass shootings, they, themselves, are rarely used in the commission of homicides. According to the FBI's statistics on homicides by weapon type, out of nearly 12,000 murders in 2014, just 248 victims were killed by rifles (of any kind). This represents a year-after-year decline since 2010 when 367 were killed by rifles. The reality is that rifles are not the weapon of choice by murderers because they are difficult to conceal. Therefore, even if we did ban all assault rifles, the impact on the murder rate in America may only be negligible.
Hillary Clinton Calls For A Ban On AR-15 Rifles After Orlando Massacre: http://uproxx.com/news/hillary-clinton-ar-15-ban-assault-weapons/
FBI — Expanded Homicide Data, 2010 to 2014, Table 8: https://www.fbi.gov/about-us/cjis/ucr/crime-in-the-u.s/2014/crime-in-the-u.s.-2014/tables/expanded-homicide-data/expanded_homicide_data_table_8_murder_victims_by_weapon_2010-2014.xls