The simple answer is to buy the caliber your gun is designed for, but if you’re just starting out, choose a paintball caliber consistent with your experience level and play style. Smaller calibers, such as .43 and .50, are meant for low-impact games and beginners. A caliber of .68 is the industry standard and is preferred by serious players, but there are even higher calibers. Keep in mind conversion kits are available to modify your marker to fire different calibers.