When a rifle with a longer barrel is fired, the force of expanding gases acts on the bullet for a longer distance. what effect does this have on the velocity of the emerging bullet? (do you see why long-range cannons have such long barrels? )

The velocity will be increased.

Explanation:

As the force will be exerted for long time on a bullet because the length of barrel is more, it will give more thrust to the bullet for a longer time hence the velocity will be more.

When bullet is shot, the gun powder is burned, so the more time it will be in the barrel, the more it will get accelerated, thus long barrel provides greater velocity to bullet. It even helps to give bullet a proper direction.