Congratulations!!!! you have the most economic propellent and a bit of historic weapon with updated version ((A rubber ))
I hope you are trying to follow all the rules of safety :
1) please do not load (the metal ball or the marble)till you are ready to shoot
2) Please do not pull the rubber till you are sure of the target.
3) Do not release the projectile till you are sure you want to hit the target.
In this case be sure what you have in front and beyond the target is something you don't want to hit.
In case of marbles be sure you do not aim at hard surfaces too close ;lest they crack off into smaller pieces and bounce off at unintended targets.
I am sure we have some experts to pool in further.
Happy and safe shooting.
Best
Vikas
Ps.In case you intend putting up the same for sale..remove the price you posted here before anyone else sees it!!!