with the powders available today the problem you are asking about is pretty much non existent you are talking maybe a 50 fps difference, 100 fps at the absolute most between a 18 and 24 inch barrel. but now if you are talking going with 57 grains instead of closer to max that will play a big role in velocity loss. with 57 grains or 60 grains of RL15 you wont beable to notice any difference in muzzle flash. also RL15 is a fast enough powder that you wont get much muzzle flash anyway now if you are talking RL19 there might be a noticebale amount muzzle flash. IMO i would just work up a good load and not worry about it at all unless you are concerned with recoil??