I'm pretty sure that muzzle flash is produced by ammunition and not barrell length. So if it means that powder type or amounts have to be modified, so be it. If there are a lot of folks shooting, even with the 5.56mm then there is still going to be muzzle signature. In fact, on my M1 Abrams tank, it didn't matter if there was no shooting being done, because my thermal sight detected body heat, recently warm engines, you name it. I think in future war, muzzle flash is going to be a non-issue. Forget my tank for a moment, and the infantry weapons like the TOW anti-tank missiles that have thermals, but in straight on infantry fighting, it won't matter if there's a lot of muzzle flash because, to put it in terms of my old southern platoon sergeant: "Them that gets hot heavy lead in the air the quickest and most accurate is goin' to be whut is the winner." Do you see my point, here? Volume of accurate fire keeps heads down. It doesn't matter if that fire is bright or dim. If your head is in the dirt to keep it from gettin' blown off, the other elements can then manuever and finish the job. The four "F" principle in action: "Find 'em, Fix 'em FIGHT 'em and Finish 'em!"
Like the muzzle flash at night from a shotgun loaded with a magnum buckshot load, that nighttime flash, blast and boom is the universal symbol in all languages for *Throw Grenade Here.*