.22 LR revolvers with cylinders long enough that a .22 mag cylinder can be substituted have a lot of bullet travel before the bullet reaches the cylinder gap, this allows fairly good velocity even if the barrel is short. The bullet moving at a fairly high velocity before leaving the cylinder.
As for barrels long enough for efficiency I'd look at the average .38 Special service revolver vs the average .38 Special target revolver. Five to five and one half inches was a compromise, but six inches was just right for maximum accuracy.
A four inch barrel is theoretically as accurate, but sight radius seems to count more than barrel length in this regard.
I'm looking at accuracy and handling qualities as the measure of efficiency, rather than maximum velocity for any particular load.