Here's some data from my rifles. Both started at 26", and I later cut them to 22". So the data is based on same barrel, same load baseline.
The 30-06 lost about 100 fps total (25 fps per inch). 60gr RL22, 210 Berger. 2800 fps to 2700 fps.
The 308 lost about 60 fps total (15 fps per inch). 49gr W760, 210 Berger. 2560 fps to 2500 fps.
And that, would be the only way to know what the actual difference would be. Same rifle, same load, same barrel but at different lengths.
