It is unlikely that you will be able to reach the same range as a gun. It all depends on what type of speargun and how strong the rubber is. For an effective distance, a good rule of thumb is to aim for 3x your speargun's length. You may find it easier to hit the targets farther away when you practice. You'll be ecstatic to witness one in action. This video is a great visual representation of spearguns shooting and how far they can be shot.