@Glennn,
You may want to update your list. You aren't including Las Vegas or the recent Florida schools shooting.
The top 6 shootings that involved a weapon classified as an assault weapon under the 1994 Federal law
Las Vegas - 59 killed
Orlando Pulse Nightclub - 50
Sandy Hook - 28
Sutherland Springs Church Shooting - 27
Stoneman Douglas School - 17
San Bernadino - 16
All of those happened after 2004 and the Fed Assault Weapons ban expired.
These mass shootings didn't involve assault style weapon but after 2004
Virginia Tech - 33
Binghamton - 14
Fort Hood - 13
Navy Yard - 13
Umpqua Community College shooting - 10
(there are no more in the top 20).
We will assume 9 for number 6
Shootings prior to 2004 not involving assault style weapon
Luby's in 1991 - 24 (2 hand guns)
Edmond Post office 1986 - 15
Camden -1949 13
San Ysidro McDonalds - 1984 - 22 (did involve rifle)
Texas tower shooting - 18 (did involve rifle)
Columbine - 1999 - 15 (did involve long guns)
The totals for the top 6 shootings involving assault style weapons - 197
The total for the top 6 shootings after 2004 not involving assault style weapons - 89
The total for the top 6 shooting since 1948 not involving assault style weapons (but including long guns) - 127.
Not only do we see that mass shootings with assault style weapons result in more casualties we see that those shootings are becoming much more common after the assault weapons ban expired.
Even more disconcerting is even though you keep claiming assault weapons are used in less than 20% of mass shootings, we see they make up well over half the high casualty shootings in the last 20 years.