What reduction to expect in a shorter barrel?

[hill billy]

If I have a given velocity in a reloading manual for say, a 24" barrel, is there a formula for reducing this per inch? I'm shooting out of a 22 7/8" barrel and would like to know where the baseline is for getting started with a new load against what the manual shows.

 

[jasonk]

Re: What reduction to expect in a shorter barrel?

Maybe 30 fps or so for the barrel length, but the manual always lie to me so I'd subtract 50 fps from what the manual says even if using the same length barrel they did.