In your projectiledata datablock, there's a line called muzzleVelocity
//projectile
...
brickExplosionRadius = 0;
brickExplosionImpact = false; //destroy a brick if we hit it directly?
brickExplosionForce = 20;
brickExplosionMaxVolume = 200;
muzzleVelocity = 20; //this line right here
velInheritFactor = 1;
...
};
You can change that to a higher/lower number for a higher/lower projectile speed.