The other firing trait is present when the barrel component is from any other manufacturer. In this case Torgue weapons will shoot Gyrojet projectiles. These projectiles have low initial speed, but accelerate as they travel forwards, and are faster than the folding fin rockets. They also deal additional explosive area-of-effect damage on impact and can score critical hits. The game calculates the bullet impact and the explosive effect separately (card only shows projectile damage), and the blast can hit enemies near to the point of impact.
Basically, Torgue guns and guns with Torgue barrels calculate two instances of damage: Impact and Explosion, using Bullet and Grenade damage respectively.