Each weapon has a special "Exclusive Power" when all of its stats are custom tuned to the maximum:

Blacktail - Increases firepower to 3.4
Broken Butterfly - Increases damage to 50.0
Handgun - Increases the chance of a headshot being fatal by a factor of five (5% or 20%, take your pick)
Killer7 - No exclusive power
Matilda - Increases magazine capacity to 100
Minethrower - Adds homing ability to mines and increases their blast radii
Punisher - Each bullet can penetrate up to five bodies
Red 9 - Increases firepower to 5.0
Rifle - Increases firepower to 18.0
Riotgun - Increases firepower to 10.0
Shotgun - Increases the effective damage range of the gun
Striker - Increases magazine capacity to 100
Semiauto Rifle - Decreases firing speed to 0.40 seconds
TMP - Increases firepower to 1.8