Availibility/Price is why everyone uses an AK-47
MP5's are often available only to law enforcement and cost $500-2500
Tec-9/Tec-22, Mac-10/Mac-11 are often used, b/c they are cheap, but they are poorly made. You can pick up a Tec-9/22 for $125-300, not sure about the Macs. They are too heavy and prone to jamming with the wrong types of ammo. Their accuracy is questionable unless you are a pro and equip them with a sight.
Uzi's are often used too, but they are only made in several places on earth. The only place you could pick them up cheap on a large scale would be Israel and the surrounding area. In the US, a full-size UZI would run you $900-3000.
AR-15/M16's are somewhat available in the US, but they have an extremely high price tag, often over $1000. The AR-15 is the civilian model and prone to jamming. If you get mud in either model, you can forget about firing it. The military uses M16's now b/c they have a slight edge in reliability. You can convert an AR-15 to an M16 by swapping some minor parts, but its a felony to do so unless you are a cop or military personnel.
AK-47/Mak-90 is available in hundreds of countries on earth. It's extremely reliable and durable. It fires ammo that is somewhat cheap, and accesories for it are cheap. They were originally made in Russia, but now are made in Russia, China, and many other places. In some countries, like South Africa, they are sold on street corners. Depending on who/what you know, you could get one for $100-650.
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