The 5.45mmx39 is a soviet
rifle round most commonly used in the
AK-74. It is not used in
submachine guns as suggested by
Jhrulith as true submachine guns only fire
pistol rounds. It is indeed used on the
Krinkov, the firearm of choice of Mr.
Bin Laden, which is a
short barreled rifle and does resemble a submachine gun.
The adoption of this round for use in the AK-74 in 1974 was most likely influenced by the US Army's decision to change their main battle rifle, the M-14 which was chambered in 7.62mmx51, to the M-16 which is chambered in the smaller 5.56mmx45 cartridge.
The theory behind using this smaller but faster "rat round" (it is actually a .22 caliber bullet, .223 inch or thereabouts) was that a hit from a bullet in this caliber would tumble upon hitting a soft target and would make nasty wounds but not always kill. A wounded soldier not only puts himself out of combat but also burdens his buddy who will have to carry him out of the battle field. End result, two or three soldiers are in effect incapacitated instead of just one. And as always, be it in battle or in self defense, we should only shoot to stay alive and not shoot to kill. Also since this cartridge is smaller and lighter, troops could carry more of it in battle and shoot at more enemy troops. This is very important when you consider that infantry men are no longer trained in marksmanship as good as they were in the old days. It's almost as if they were trained to Spray and pray.