Well it's a bit smaller than half an inch but what's really important is the weight of the projectile and clay balls are the lightest ammo you can shoot pretty much so there is going to be a lot of wasted energy everytime you take a shot. When I shoot clay that same size I use .4 - .5mm thick bands. I saw these .6mm ones on Amazon, they would be a much better fit for 10mm clay balls than the .75mm thick bands.