Would there be a significant difference in the muzzle velocity?

Let us suppose that you had a revolver and a single-shot pistol in the same caliber, firing the same type of ammunition, through the same length barrel. If it helps you to visualize, imagine a Ruger GP-100 and a Contender, both in .357 Magnum of course, with 6-inch barrels. Does the cylinder gap make much difference?