Why do longer barrels increase projectile velocity in superguns?

[Soaring Crane]

Iraq was reportedly developing a “supergun” with a barrel some 70 m long, which was to be able to launch huge artillery shells several hundred miles. During WWI, Germany used a “Big Bertha” cannon to hurl shells into Paris 30 miles away. This gun also had a long barrel. What is the reason for using a longer barrel in these guns?

a.To reduce frictional losses.
b.To allow the force of the expanding gases from the gunpowder to act for a longer time.
c.To increase the force exerted on the bullet due to the expanding gases from the gunpowder.
d. To reduce the force exerted on the bullet due to the expanding gases from the gunpowder.
e.To provide a more favorable ratio of kinetic energy to potential energy.

I do know that a greater velocity and, therefore, greater kinetic energy results from a longer barrel. Could it possibly be E.?

Thanks.

 

[DaveC426913]

Wouldn't a longer barrel maximize the acceleration of the projectile from the charge? The moment the projectile leaves the muzzle, the charge dissipates and the projectile goes into ballistic motion. A short barrel would waste all that charge.

 

[Soaring Crane]

I think the acceleration increases but I am unsure of what exactly happens in the projectile (from what I read from this article):

http://home.howstuffworks.com/revolver2.htm

"The inside of the barrel is lined with spiraling grooves, which spin the bullet to give it stability. A longer barrel improves stability, since it spins the bullet for longer. Extending the barrel also increases the speed of the bullet, since the gas pressure accelerates the bullet for a longer period of time."

 

[Doc Al]

That quoted passage should make it clear which answer is correct.

 

[Soaring Crane]

From the excerpt, would the answer be b. in which the gases act on the shell for a longer time period?

 

[Doc Al]

That's what I would say.