Just a brief bit of data to help you select your ammo. With spheres, a 25% increase in diameter doubles the volume. So moving from 8mm to 10mm doubles the volume of your projectile, which of course doubles the weight. Next item is density. Here are a few: Glass: 2.4 Steel: 7.82 Lead: 11.35 Next is impact characteristic. Glass will either bounce off a target or shatter.... but due to its low density, will not have enough momentum for those shards to do much damage. Steel, at slingshot velocities, will bounce off virtually any target. A bounce does not impart the total kinetic energy of the projectile; the distance of the bounce illustrates how much energy has been lost. Lead will deform on impact and bounce far less than steel, thereby transferring more energy to target. Preparing the projectile by making particular cuts will help it deform even more, enhancing the deformation and thus maximizing the energy transfer. So then. Marbles are fun to shoot, but ineffective against targets. 8mm steel, which is common ammo, is good. But .50 caliber lead, while the same size as the marble and not much bigger than the 8mm steel, will weigh nearly three times the steel (and almost 5x the glass marble). This gives it far more energy, more momentum.... and as it deforms on impact it will transfer that energy to target much more effectively. Why am I writing all this? Because I’m going to build a thing based on “instant Rufus” using .50 lead. The thing needed to utilize lead is a spring-loaded in-barrel retainer. Doesn’t need to be much, just enough to keep the lead in the breech. The ball and the follower will both easily push it out of the way upon trigger release, and upon reset. Btw, the retainer will also allow pump-action with marbles.