Paintball

A paintball is a small hollow sphere filled with a non-toxic, washable 'paint'. They are designed to splatter upon impact.
They are commonly available for the game of paintball, and they are round and have a fairly uniform mass - generally good for accuracy in smoothbore barrels.

Aerodynamics

Paintballs are assumed to have a Cd of about .5, as they are smooth spheres.
They may be launched out of a curved barrel to increase range via the Magnus effect (the same at work in hop-up devices).

Use and target performance

Paintballs are typically launched out of 3/4" CPVC, as that is the most commonly available that pipe that is a close match for their diameter. Some prefer 3/4" sch 80, and the ultimate is recognized as being 3/4" sch 120, although this pipe is extremely hard to find and not commonly used. Sch 120 barrels are thicker and are a tighter fit, decreasing sag and increasing velocity.

Paintballs are typically breech loaded, and their shape allows them to be stored in magazines/clips.

Upon impact with:

• A twig, a paintball typically ruptures.
• A water-filled milk jug, a paintball typically ruptures.
• A tree, a paintball ruptures.
• A rock, a paintball ruptures.
• A person, a paintball typically ruptures, resulting in laughter (or serious injury - see below!).

Due to their low sectional density, most launchers will fire a paintball too fast to be considered safe, which is generally considered to be 300 fps. Furthermore, actually making a reliable, accurate, and rapidly firing weapon is a complicated task that probably can't be done without a lathe.
Even then, most fields won't let you play with your homebrew weapon; spud guns that fire paintballs should be considered target-practice devices only.