Additional levels may be acquired through the use of a Champion, Heavy Gunner, Rifleman, or Support Gunner Class Mod.
All weapons that fire bullets, including shotguns, receive the bonus damage. Rockets, grenades, and Eridian energy-based projectiles do not receive the bonus.
Impact is unique in its mechanics in that it does not increase Roland's damage, but rather increases the bullet's damage. What makes this interesting is that since different stats are affected, the end result is Impact's bonus is added in a multiplicative (as opposed to the traditional additive) fashion. Because nothing else provides extra bullet damage, Impact gives a "hard" +3% bonus per level, regardless of proficiency, skills or Class Mods.