$800-$2,000.
From 2013-2015 it has been the AR15.
You can purchase some AR15 parts from places such as OpticsPlanet, m4broker, Tactical Intent, Cabela's, and you should also check some local gun shops and gun shows for some accessories.
3 Gun Nation - 2011 AR15-Com Pro-Am Part I 3-7 was released on: USA: 26 September 2012
100-1000 USD
ARmalite Model 15 but it very commonly thought to be automatic rifle.
Yes, most assault rifles are illegal, unless granted special permission and you have acquired a special gun license from the Cayman Islands Gun Club.
First do a full functional clear, eject the mag, rack the bolt/carrier to eject any casing. Next fully clean the gun. Check for broken or weak parts in bolt/carrier, gas system, and buffer system. Use quality ammo. If it still doesn't then take the gun to someone who knows how to work on ar15 rifles.
It is the Carbine (short rifle) version of the AR15. AR stands for Armalite Rifle. It is a semi auto rifle.
There are a few websites where someone can find detailed schematic gun pictures. Some examples of websites where someone can look are Forbes, AR15, and Take Down Guides.
Depending on condition and configuration, a few hundred to a few thousand.
AK47 M-16 AR15 1911-a1 BAR