Shooting 3/8" = 9.525mm steel ammo. Noticed some tournament rules limit ammo size to 9mm or less. Is that the case for most contest shooting? 5/16" =7.9375 mm looks to be the next size down available from most AMZ sellers. Who, what if it exists is the governing body for tournament slingshot shooting? Rules and regs can become restrictive and limiting. The open nature of slingshot shooting is one aspect that make it personally attracting.
But for competitive shooting you need defined parameters to ensure it's the shooter vs. the hardware that determines accuracy. Having a set of common rules for competition at the local to World Championship level would allow for proper training. Also for comparison even if you never travel beyond local events if there is a handicapping/ranking system.