Two types of barrels were explained in the last step. There is a third type, the barrel-hopper combo.
A hopper is basically a compartment that holds ammunition until the ammunition is ready to fire, and the hopper will feed it into the barrel. The hopper is called many things-hopper, magazine, clip, etc.-and each one is a slightly different part on a real weapon, however, they are all accepted as the same thing in the world of K'nex.
One example of a good hopper-barrel combination is crestind's AST Rifle.
Another great combo is Ipod Killer's Sniper Rifle
The best combination is probably the barrel of Perfect Duck's Red Impact
because it lacks the friction that most hopper-barrel combinations have.