The AKMSU (АКMСУ, Автомат Калашникова Mодернизированный Складной Укороченная, Avtomat Kalashnikova Modernizirovannyy Skladnoy Ukorochennaya, Kalashnikov's shortened modernized assault rifle with a folding stock), also known as the AKSMU, is a rather unknown variation of the famed AK assault rifle.
Little is known about the real history of the AKMSU due to many conflicting accounts; some say that it was a one-off prototype manufactured in the Soviet Union, while others say it was what is known as a "Khyber Pass copy", a weapon produced in Pakistan's Khyber Pass, an area which is known (or notorious, depending on how one looks at it) for diverse and unique production of weapons in small shops.
The AKMSU is "made" by combining an AKMS receiver with an AKS-74U gas system, receiver cover and front trunnion. The prominent muzzle brake of the AKMSU is actually required for the rifle to correctly function, as it contains an expansion chamber which helps to correctly cycle the rifle and has multiple grooves machined into it for heat dispersion. The "AKMSU" in the United Kingdom's Pattern Room and the one photographed the most isn't an actual AKMSU; it is actually a Chinese Type 56-1 assault rifle combined with an AKS-74U trunnion dated from 1977. The lower handguard may well be the only authentic piece from a real AKMSU, and while no pictures of a real AKMSU are known to exist, many sources do attest to its existence.