There was a piece of strong twine attached to a nail in the center of the "muzzle" notch, then laid back atop the gun in the groove until it extended out through the "rear sight".
Then, starting from the muzzle end, "rubbers" were stretched from the muzzle notch back to the top notches, each one trapping the twine beneath it.
When you had the 22nd "rubber" stretched back to the notch in front of the rear sight, you were "ready to go to war"...
Then, simply by pulling upward on the loose end of the twine at the back, you could loose single shots or "hose 'em good"! Of course the bands at the front had considerably lower "muzzle velocity"...
But, for a while, there, I was the envy of the whole neighborhood -- after the "coping-saw blisters" healed, that is... '-}