Watch the muzzle flash of a carbine versus rifle in same caliber. A Colt Officers Model versus standard full size [1911] Colt, or any short versus long barrel revolver in same caliber. Greater flash means powder still burning on exit - expending energy into the air rather than pushing the bullet.
But isn't the argument moot?
At the appropriate range, and for the purpose intended, each is "best" and "power" comes more from the bullet mass than from a couple of degrees of muzzle velocity.
[Meaning that you're probably gonna feel 'more hurt' by a .45ACP from a three inch barrel at ten feet than by a significantly faster .22 from a twenty-plus rifle barrel at same range...but you can't factor in a lucky shot or a stress induced miss.]
Probably strictly true, but not practically speaking. Why bother to load powder that's not going to be burned anyway? Load less but faster burning powder. Handgun ammunition is designed to be fired from handguns, rifle ammunition in rifles. Both are designed to be used in the most typical barrel length. Reloaders can tune their loads to a particular length, and even to a particular gun.