Calculating Force: The Physics Behind a Rifle's Shot Speed and Barrel Length

[atse1900]

A 5.0 g bullet leaves the muzzle of a rifle with a speed of 320 m/s, What force (assumed constant) is exerted on the bullet while it is traveling down the .82 m long barrel of the rifle?

This is what I have so far. : [.

(320)^2/(2*.82)=t

320/t=a

a= 62439

62439*.005
=312

The back of the book is 310. What did I do wrong?

 

[Homer Simpson]

I got what you got.

 

[andrewchang]

probably because the book gives answers in significant figures?