Axis: The bullet will travel along the axis of your choice. It's the shooting direction.
Pick Base: Pick objects as bases. The bullet will not collide with these bases.
Pick Bullet: Pick an object to be the bullet.
Multiple Force: multiply the force value, in order to explode high mass targets
Delay for Auto: The period of time between each shot when Auto Shoot is enabled.
Hint
Make the center of both barrel and the bullet their pivot to ensure that the bullet will not fly through the barrel. The pivot point of the barrel is the starting point of the bullet.