Why do longer barrels mean faster bullets, if the pressure decreases the longer the bullet stays in the barrel?
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So the way a bullet will leave accelerate down the barrel is because of pressure. The explosion is just expanding gas and so you can think of it like a balloon. As air leave the balloon it will push the balloon further away. As the air leave the balloon pressure will drop, but as long as there is still pressure, it will keep pushing the balloon. It will just push less and less until all the air is out and the pressure between inside the balloon (inside the barrel) and the outside world is equal.
In the case of a bullet you will have mainly 3 forces on the bullet. The pressure for the explosion, the friction of the air in front of the bullet and the friction between the bullet and the barrel. As long as the pressure exert a bigger force than both friction, the bullet will keep accelerating.
So even if a longer barrel will increase the velocity of the bullet, there is a limit to that effect.
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