Start by reading this article Why I "Need" and AR-15
Unfortunately, mass shootings occur just about every week these days and calls have gone out to ban the AR-15 again from civilian sales. The main arguement being that civilians don't "need" that much gun for hunting or protection. Jon Stokes, the author of this article, thinks otherwise and makes some great points.
"The AR-15’s incredible flexibility, accuracy, and ease-of-use combine with its status as the most thoroughly tested and debugged firearm in military history to make it massively popular with shooters of all stripes, from hunters to home defense buyers to competitors to police. Parts for the AR are available literally everywhere, and the Internet is chock full of maintenance information and training videos."
"This is all part of the reason why I, a civilian, “need” a military-grade combat weapon. I don’t want to shoot and miss; I don’t want the gun to jam because it’s dirty or cold; and I don’t want to hit my target and then have it run off into the woods and die lost and wounded because I didn’t “bring enough gun”. Like my grandpa with his “military-grade” lever action rifle, I want a modern firearm that’s popular (which means parts and training are cheaply and widely available), ergonomic, rugged, accurate, and reliably effective, so that none of the aforementioned bad things happen to me when I’m shooting."
He's right. The AR-15 is the most widely used and advanced gun for a reason. It is reliable, modifiable, and easy to use. Thats why everyone uses it - military, police, hunters, and terrorists.