That rifle that FN makes uses the smaller 5.7 cartridge. The Chinese are also using that same concept for many of their firearms because it allows a soldier to carry a LOT MORE ammo. Keep in mind that even the small rounds WILL kill you and are very accurate too. Think two to the chest and one to the head for close in combat. The link below shows you what the Chinese are thinking about with their firearms...
I've fired both the FN Five-Seven pistol and the PS90...both very good guns, however, the pistol is absolutely stunning. For a pistol it is extremely accurate (about 2" groups if you are good) out to about 100 meters. For the power of the round, well...it is extremely effective, they have armor penetrating rounds which will penetrate level IIIA body armor and helmets. The round itself tumbles once entering the body just like a .223/5.56 and causes an extreme amount of damage. If you have the money for either of those (and the money to buy alot of the ammo since it is really expensive), then I would highly recommend them both.
They have actually come up with a conversion for the AR lower to use an upper for this round. It uses the same 50 round magazine. The used casings are ejected out of the magazine port. The downside I have heard is that the standard rounds are on the low side for stopping power.
I was in a gun shop the other day and they had an AK bull pup for I think 475. I was going to look at it but really didnt have the time so I just picked up some ammo and left. The AK looked neat but not sure I would like it.