AK-47 denotes the Automatic Rifle designed by Kalashnakov in 1947.
Very few AK-47s are made any more because it has been replaced by the AKM, which stands for AK...Modern! But it is pretty much the same thing so most people call both of them AK-47s.
Now, the Soviets saw how good our 5.56 ammo was in Vietnam, and decided to copy it by necking down the 7.62x39 to 5.45x39. (This is kinda funny because the soviets were very impressed with how deadly it was, but many of our own troops thought it wasn't deadly enough. I guess it goes to show you the grass is always greener on the other side, whatever you are getting shot at WITH is always much feared and whatever you get to shoot back at them with is always coming up short)
Anyways, this new AK in 5.45x39mm was called the AK-74.
Now a shortened version of that called the AKS-74U or sometimes written AK-74SU was created, basically the soviet versio of the M4.
Some people say that the S desingates a folding stock and the U designates a shorter barrel. However, there were plenty of AK-47s and AKMs with folding stocks that didn't get a special letter designation.
In theory someone could shorten an AK-47 in the same way and call it an AKS-47U, but to my knowledge no major manufacturer or any goverment makes such a gun.
Enough knowledge for today.
