Pictured on top is a seldom seen Sheridan Model G CO2, .50 cal dart gun; circa 1978. There was also a pneumatic version made, and both CO2 and pneumatic pistols. The gun pictured at bottom is a Benjamin/Sheridan Model 176B. I believe this model replaced the Model G in 1992 and is still produced today by Crosman.
The Model G is a muzzle loader and the 176B has a bolt that swings to the side for loading. Power on both Models can be regulated with the knob on the rear. Practice darts are available to shoot in these guns and .50 cal paint balls may also work just for the fun of it.