Here's the differences between the AR-15 (the civilian model) and the M-4 (the smaller, military equivalent).
The M-4 is capable shooting a single semi-automatic and a 3 round burst (where one pull of the trigger fires three bullets). The civilian AR-15 can only shoot semi automatic (one trigger pull equals one bullet, but the gun auto chambers the next round).
The common thing you hear is that the AR-15 isn't an assault rifle because it doesn't shoot full auto. People like to ignore the fact that members of the military rarely shoot on full auto, as it decreases your accuracy and wastes ammo. I was in the military for 12 years and the only time I ever shot burst was in training just to see what it could do. Its not something you want to rely on in a firefight as it will decrease your accuracy and waste your ammo. When I was in Afghanistan, I carried 4 magazines with 30 rounds each. Those 120 rounds would be the difference between me living and dying if I got in a firefight (thankfully, it never happened). You want to make every bullet count, burst mode takes that away
The civilian AR-15 is a modified version of the M-16 to make it so that you can't swap the parts that enable the three round burst mode. Colt specifically designed the AR-15 so the lower receiver from an M16*/M-4 can't be used on an AR-15 (although it's not that hard to get around if you know what you're doing) The vast majority of other accessories, including magazines, are swappable
Other then that, they're the exact same weapon. These guns are efficient at what they're designed to do because they were designed with the intent to kill people in a combat situation. They're designed to allow the user to fire a lot of shots off in a short amount of time, empty the clip/reload, and get back to firing as quickly as possible while in a firefight. The last thing you want to do is take to long to reload if someone is shooting at you. That's why the AR-15 has been the weapon of choice in shootings like this. You can carry a lot of ammo, it's quick to reload and it allows you to get a lot of shots off in a short amount of time.
The AR-15 is an assault rifle and anyone who says otherwise is full of shit.
And yes, the dipshits who treat their guns like an extension of their penis are out of their minds.
*The M-16 is very similar to the M4, just with a bigger barrell and is a bit heavier.
