I have done a lot of reading about this subject and it eould appear that the answer is not as much as you may think.
Shooting times did some tests taking a rfa quadlock 24" barrel and slwly reducing its length to 18".
The conclusion was, starting with 150gr sierra match king doing 2905 fps reducing the length to 18" and using the same load gave 2748 fps, a loss of 157 fps.
There are many other studies posted on the internet which say basically the same.