Why do longer barrels mean faster bullets, if the pressure decreases the longer the bullet stays in the barrel?
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The easiest way to think about it is that energy is equal to force times distance. Yes, the force decreases as the bullet moves along the barrel, but there is still force being applied over a distance, so more energy is being added. This only stops being the case when the pressure behind the bullet is equal to atmospheric pressure (ok, a bit more since friction exists). I don’t think any real guns use barrels long enough to extract all energy from the powder, since you reach a point of diminishing returns pretty quickly.
As long as there is still enough energy to accelerate the bullet appreciably, the additional length is still adding velocity. There is a point where the gasses’ expansion slows enough that it’s no longer applying any force to the bullet, but that happens significantly after the peak force.
Think about it like flooring your car from a stop, you can slowly let off the pedal but still be accelerating for a lot of that range, it’s not a straight max or off arrangement.
It takes time to change the energy from the gunpowder explosion into bullet movement, and the longer barrel means there’s more time for that to happen. As long as the explosion is pushing the bullet, it is adding energy to it. The moment the explosion starts pushing the atmosphere instead of the bullet (the barrel ends), the bullet stops gaining energy.
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The longer the barrel or the added suppressor, the longer the time the bullet have to attain energy from the untouched explosion force by the surroundings in or out the barrel/suppressor moving behind the bullet accelerating it.
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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