The flash-suppressor on a 15-22 isn't the same as one on a 5.56/.223 MSR, in that it doesn't have the "solid" bottom area (that needs to be "timed") so it doesn't make a dust cloud when you're shooting from the prone position. Smith&Wesson probably figures that the muzzle blast from a 22LR isn't going to make a dust cloud, and they would be right.