If you're looking for another fairly accurate method of shooting a rubber band, and you'd like the convenience of firing with one hand, here's another way.
Hold out your hand of choice. Hold it straight out, palm up, all fingers out. Now, fold in your middle and ring fingers.
Take said rubber band, and wrap it around your pinky. Fold your pinky in, so the rubberband cannot escape. Turn your hand ninety degrees inward toward your body so that your thumb stands straight up, and your index finger points forward.
Pull the free end of the rubber band out, and around the outside of the joint of your thumb, and pull to the tip of your index finger. Wrap the inside of the free loop of the rubber band onto the tip of your index finger so that it rests there.
Good. You have now armed your rubber band pistol. Your fingers should be formed into the traditional "gun" position. "BANG BANG, YOU'RE DEAD!"
While keeping your rubber band securely positioned, point your index finger at your desired target. When you're ready to fire, simply open your pinky enough for the rubber band to unhook and escape. THWAP!