Barrel length vs. FPS

Anyone know how a given formula for figuring out the how much FPS is lost or gained by the lenghth of the barrel? Or better yet..... I've just aquired a Tikka T-3 lite in .308 win, I think the barrel is around 23 inches or some odd-ball lenght like that. My gunsmith,who I usually trust says for a few bucks he could take a couple inches off the end and re-crown if I'm really wanting to save wieght and It wont hurt the FPS to much....in fact not much at all. Is this true? I know there's factory made .308's that have been made with shorter barrels than this but what I'm wondering is if I'm going to hurt the calibers ballistics by cutting 2 inches off which should make it about 21 inches give or take?

The average is about 25fps per inch of barrel. That is a average, some high velocity mags will lose more. Your .308 should lose about 50-70 fps for removal of 2 inches. Not much to worry about.
RELH

RELH.....Thanks, I was hoping I would get a response from you. I've really come to appreciate and respect your opinions and expertise. Thanks for sharing it with everyone. Ryan