I want to "hit" a fixed spot with a physical object over relatively short distances (< ~20 ft).
This must not present a danger to nearby bystanders (in the event of a "misfire"). "Weapons" are out of the question.
And, the object must be of sufficient size to be clearly visible in transit. This also suggests a low transit velocity.
I figure I need a bit of mass to ensure aerodynamic effects don't bugger the calculations. E.g., a softball would be better than a softball-sized hollow ball which might exhibit more nonlinear behaviors as it transits from projectile to ballistic motion.
The target is (effectively) a "spot on the floor". I.e., not a vertical "hoop" to pass through (like goalposts in soccer).
Accelerating a significant mass would likely prove to be a challenge so "lobbing" the object seems more practical. It would also *seem* to be more tolerant of aiming issues than something HOPING to travel in a straight line (like a bullet).
So, conceptually, a tube ("barrel") to guide the initial segment of flight and some sort of mechanism to propel the object from the tube.
Pneumatic, hydraulic, mechanical, etc.
Will the control over the propulsive force be the tougher challenge or the precise aiming of the launch tube?
[You are given (r,theta) to target and no feedback as to proximity of strike -- unless a direct strike. The target -- or launcher -- will move after each attempt]Of course, "you" is a machine...