I took the design from the Minecraft Wiki (http://www.minecraftwiki.net/wiki/Tutorials/TNT_Cannons) and made a double layered design. Extremely simple, light, and uses few materials. But it fires like a rifle.
One day, this will be massive (maybe) but I decided to share a very simple, but effective design. There's no instructions to how to reload it, but plenty of ammo. Nothing really you're firing at, but a mountain.

Additional Notes

Texture pack: Misa's Realistic 64x64