Some 4 or 8 years back, either Guns & Ammo or Combat Handguns did an article on this very subject. They used (as I recall) a Ruger, starting with a 10" barrel. Disremember the bullet weight, but I think it was factory stuff.
Beginning muzzle velocity, I think, was around 1,400-1,500 fps. 240-grain? Anyway, they cut back by one inch at a time and shot over a chronograph. The decline was fairly straight-line, at around 40 fps per inch.
They startled themselves by firing a round with no barrel at all--and chronographed over 1,000 fps! (1,054 fps sticks in my mind.)
Let me generalize from various articles over a bunch of decades (I have The American Rifleman back to 1940): In general, if you know the length of barrel for a published muzzle velocity, for pistols you can add or subtract around 40 (maybe 50)feet per second per inch of barrel difference.
Rifle ammo-data is generally based on 26" barrel-length. Most articles indicate a loss of 70 feet per second per inch of barrel less than 26". Now, for some cartridges this does not hold. The .308, for instance, with Win. Ball Powder, was supposedly designed for optimum efficiency in a 19-inch barrel...
There might be an index at the NRA site on this. I know there have been articles on this subject, off-and-on, since the '30s...
Hope this helps, Art
