Well I have it all figured out and I'm not sure why I couldn't before...
You use the velocity and distance to calculate the time it takes to leave the barrel with this formula
d = 1/2 ( Vf + Vi ) × t
You then use the time to calculate the acceleration using this formula
a = ( Vf - Vi ) / t
Now that you have acceleration, you can use
F = m a
just remember that the mass of the bullet has been given in grams and thus needs to be converted to kilograms
When you are calculating acceleration, you WILL get a big number
but it gets more normal when you calculate force because the mass of the bullet is so small.
Note: This was just copypasta'd from my facebook chat explaining to a friend.