I'm using a pump for my Discovery, and I need the exercise. A dive tank is 80 cubic FEET of air at 3000 psi. You are looking at a tank of 72 cubic INCHES. I don't know how many cubic inches of air at 2000 psi goes into an air rifle, but the tank will rapidly run out of peak pressure, and you will be paying to refill it again and again.
I'll have to let more experienced air gunners weigh in on the merits of such a small tank.
Gordon