Well, back in the good old days, an acquaintance of mine had an Colt XM177E2 submachine gun. Someone had "borrowed" the special "sound and flash suppressor" that was on the barrel. This fatter suppressor had an inner wall and outer wall; the inner wall had a series of holes for the the muzzle flash to bleed into and there were six slots around the front end of the suppressor. This device did control the muzzle blast of the 11.5-inch barrel and did make a reduction in the sound signature below that of the 20-inch rifle barrel.
However, since the specialized piece was missing, a conventional flash suppressor from a standard M16A1 rifle was installed. In bright daylight, the muzzle blast and flash was a fireball about three feet in diameter. Heaven only knows what it must've looked like at night!
The 5.56x45 NATO M193 (55-grain) or M855 (62-grain) Ball round is designed for a 20-inch barrel. Shorten the barrel and you will have a huge muzzle flash (and blast) caused by the burning powder. The shorter the barrel, the worse it gets. It is that simple.