system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What is the point of a gun barrel? Especially long ones. Wouldn't it be better to make the barrel as short as possible?
gpt
When the bullet leaves the barrel, the expanding gas behind it can expand in all directions, instead of pushing the bullet forward. So the longer the barrel, the more time the bullet spends accelerating (to a point, of course: at some point energy of the gunpowder is used up, and friction with the walls of the barrel starts slowing the bullet more than the hot gas accelerates it).

Also, a longer barrel has more time and distance to make sure the bullet is going in the right direction. If we imagine the opposite case, where there's no barrel at all, just an explosion out in the open with a bullet sitting next to it, it would be extremely hard to make sure the bullet gets thrown in the right direction.