You are correct in some way. There is a reason why your loading books tell you the length of the barrel when publishing their books.
Are you telling me that a 10" barrel will push out the same bullet/load as fast as a 22 or 26" barrel??
Nope I don't think anyone is trying to tell you that.
However the powders used that produce the best combination of Velocity and Accuracy in those long rifle barrels will still produce the best combination of Velocity and Accuracy in 14-15" Handgun Barrels, just at a lesser velocity level.
In regards to cartridges such as the .223 Remington velocity loss from a 22" Rifle to a 14" Handgun will be roughly 200 - 400 FPS depending on the individual barrels used, bullet weight etc. I have also seen where going from a 14" Contender Barrel down to a 10" Contender Barrel will result in as much velocity loss as going from a 22" Rifle Barrel down to a 14" Handgun Barrel, again per the chronographing I have done.
In the .243's I have tested velocity loss from a 22" Barreled Rifle to a 15" Barreled Handgun have been in the neighborbood of 350 FPS with heavier bullets to up to 500 FPS with 55gr. Nosler Ballistic Tips from the chronographing I have done.
Granted these were all different barrels and I did not start with a Rifle barrel and cut it down, so differences in chamber and bore dimensions can have an effect on the outcome.
Larry