notyou2 wrote:I would have thought the Kalashnikov was the world's most popular light machine gun.
The AK-47 is an assault rifle. The Uzi is a light machine gun.
The basic difference is that an assault rifle uses medium-powered ammunition (cartridges longer than pistol cartridges, but shorter than the long rifle cartridges used in a hunting rifle or a sniper rifle.) Although having less recoil than long rifle ammo, assault rifle ammo still produces enough kick that the rifle needs to be cradled against the shoulder like a long rifle.
A light machine gun (sometimes called a machinepistol) uses very low powered (pistol sized) ammunition. These low-powered cartridges are intended for close-quarters combat where range is not an issue. They produce so little recoil that the gun doesn't need to be cradled against the shoulder, and can be fired in a wide variety of postures, including while running around and flailing about wildly as so often happens in close-quarters combat.
For a fuller discussion of the difference, see:
http://firearmshistory.blogspot.ca/2010/11/what-is-difference-between-rifle-slr.html