Yep I think anything above 26" will act like a long gun. Anything under 26" will be a bit more like a pistol.
The time span would also be important as far as powder burning capability was concerned. Prior to about 1820 much of the powder was serpentine powder, was it not? - mixture of ingredients. About 1820, there was a switch to more highly refined powder in the military, which is where most civilian stores on the frontier would be obtained. (just speculation)
Prior to about 1820, military ctg. in the US was 165gr. of mil-spec powder (included priming) with a .64" ball, for the .69 cal. muskets, which ran about .69 to .70 cal. After 1820 or thereabouts, the charge was dropped to 135gr. which included priming, with an increased ball size to .65". The first .69 cal. Caplock musket was the model 1842.
I would surmise that after about 1820, shorter barreled guns would perform better than those pre-1820.
Of course, how well would a smoothbore perform in any barrel length - close-range guns that they were.