@sceletera,
Okay, we'll look at all of the mass shootings since 2004, after the federal ban expired. And yes, I've included your list of six.
Since 2004:
Orlando nightclub: 50 killed. Both rifle and pistol used.
Sandy Hook: 28 killed. Both rifle and pistol used.
Sutherland Springs Church: 27 killed. Rifle used.
San Bernardino: 14 killed. Both rifle and pistol used.
Orange County, Florida: 5 killed. Pistol used.
Fort Lauderdale airport: 5 killed. Pistol used.
Roseburg, Oregon: 9 killed. Pistol used.
Chattanooga, Tennessee: 5 killed. Both rifle and pistol used.
Charleston, South Carolina: 9 killed. Pistol used.
Isla Vista, California: 6 killed. Pistol and knife used.
Washington, D.C.: 12 killed. Pistol and shotgun used.
Santa Monica, California: 5 killed. Rifle and pistol used.
Minneapolis, Minnesota: 6 killed. Pistol used.
Aurora, Colorado: 12 killed. Pistol and rifle used.
Oakland, California: 7 killed. Pistol used.
Seal Beach, California: 8 killed. Pistol used.
Tuscon, Arizona: 6 killed. Pistol used.
Manchester, Connecticut: 8 killed. Pistol used.
Minnesota: 9 killed. Pistol used.
Goleta, California: 6 killed. Pistol used.
Meridian, Mississippi: 6 killed. Shotgun and rifle used.
Nickel Mines, Pennsylvania: 5 killed. Shotgun, bolt action rifle, and pistol used.
Salt Lake City, Utah: 5 killed. Pistol and shotgun used.
Blacksburg, Virginia: 32 killed. Pistol used.
Omaha, Nebraska: 8 killed. Rifle used.
Dekalb, Illinois: 5 killed. shotgun and pistol used.
Binghampton, New York: 13 killed. Pistol used.
Fort Hood, Texas: 13 killed. Pistol used.
http://timelines.latimes.com/deadliest-shooting-rampages/
____________________________________________________________________________________________
I split the number on those killings done with both rifle and pistol. It comes out to 95 killings done with rifles, and 212 killings with pistols. It does not appear that rifles are preferred by mass shooters.