USA Today has an effective graphic database on mass killings between 2006 and 2014.It shows, for example, that 887 of 1,172 deaths were from gunshots and 285 were from other weapons, such as fire, beatings, or drownings.
The first thing that I looked for was the use of assault rifles in mass shootings. They do not have such a list, but they have something close. The results show that 72% of mass shootings were committed with a semi-automatic pistol or a revolver. An automatic rifle was used in only 0.4% of the incidents. Since “assault weapon” is not a well defined term, there no category for it, but it would certainly be among the semi-automatic rifles. If we assume that all semi-automatic rifles are “assault weapons”, and add automatic rifles and automatic pistols, we find that less than 10% of mass shootings used an “assault weapon” or fully automatic weapon. They were also used in less than 7.5% of mass killings overall.
Using the FBI crime statistics for 2006-2010, for 2011-2012 and assuming that the number of homicides in 2013 was the same as 2012, it appears that mass shootings using an “assault weapon” or fully automatic weapon constitute less than 8 hundredths of one percent of all homicides over the 2006-2013 period.