That is purely a myth and a common misconception. Powder burn rate is chosen for the desired pressure range and capacity, not barrel length. Powder does not burn all the way down the barrel. It is consumed by the time the bullet reaches the forcing cone. Slower burning powders are used in greater volume and produce a greater volume of expanding gases. Those expanding gases venting to the atmosphere are what cause muzzle flash, not burning powder. Thus the same powders that yield the highest velocity in a rifle length barrel will do the same in a handgun length barrel. The .30M1 is the birthplace of H110. What powder commonly yields the highest velocity in revolvers? H110/296 and those with a similar burn rate.