When a rifle with a longer barrel is fired,the force of expanding gases acts on the bulletfor a longer distace

Question

what effect does this have on the velocity of the emerging bullet? Why do long range cannons have such long barrels?

Answer 1

What you said is absolutely right. This factor alone would make the velocity of the emerging bullet higher. The same argument can be applied to long range cannons.
But the barrel can not be elongated indefinitely. For one reason, the friction between the bullet and the barrel will also act longer. For another reason, too long a barrel for the guns/ cannons will change the gravitational center.