Shooters/attempted shooters who modified the function of their guns

Eric Harris - Sawed Off Shotgun
Dylan Klebold - Sawed Off Double Barreled Shotgun
Sky Bouche - Sawed Off Shotgun
Mason Campbell - Sawed Off Shotgun
Derrick Bird - Sawed Off Double Barreled Shotgun
Matthew Raymond - Sawed Off Shotgun
Jan Molenaar - 2 Sawed Off Shotguns
Charles Whitman - Sawed Off Shotgun
Aaron Alexis - Sawed Off Shotgun
Raoul Moat - Sawed Off Shotgun
Alvaro Castillo - Sawed Off Shotgun
Man Monis - Sawed Off Shotgun
Alexander Onufrienko - Sawed Off Double Barreled Shotgun
Kevin Addison - Sawed Off Shotgun
Malcolm Baker - Sawed Off Double Barreled Shotgun
Anders Breivik - Sawed Off Shotgun
Robert Poulin - Sawed Off Shotgun
Frank Vitkovic - Sawed Off M1 Carbine
Kipland Kinkel - Sawed Off Ruger 10/22
Sebastian Bosse - Sawed Off .22 LR Bolt Action Rifle Sawed Off Percussion Rifle
Matthew Murray - Altered Bushmaster XM-15 to shoot 6.8mm Remington SPC
Eduardo Sencion - Altered MAK-90 to Fully Automatic

San Bernardino Shooters - Tried and failed to turn one of their guns full auto
John Zawahri - Modified his black powder revolver to fire .45 long colt
Raymond Spencer - converted his rifles to full auto
Artem Kazantsev - converted blank pistols to take live ammo, or maybe bought them already converted? No idea

Steven Kazmierczak sawed off the barrel of his shotgun.

Robert Bowers converted his .357 SIG Glocks to .40 S&W.