Put simply, it's a barrel that launches metal projectiles called bullets using the explosive force of gunpowder.

Correct. Essentially, he used his bare hands to form the cannon's barrel.

When Gon launched his ball, Killua barely used any protective Nen on his hands.

And I can do it faster than Killua. In the instant before Gon touches the ball, you can apply a lightning-fast coat of aura to your hands for protection!