The standard configuration for an AR-15 uses a 16" barrel. This is often referred to as carbine length. However, there are some cases where it is advantageous to have a shorter barrel length. The main reasons being to save weight and space when using indoors.

Once you go below 16" it must be registered on a form 1 with the ATF as a short barrel rifle. You may choose any length but the most popular choices are 12.5", 10.5", and even 7.5" in some cases. It is important to know the shorter you go in barrel length, the less velocity you will generate. In addition, you may need to use heavier bullets as they tend to remain stable with less velocity. I'm not saying you can't shoot a 10.5" barrel out to 300 yards because you can. It just won't be as consistent as a 14 or 16 inch barrel.

One of the biggest downsides to having a short barrel is the amount of muzzle flash/report. A 7.5" barrel is brutal to fire indoors, and generates a report similar to a flash bang. In addition, the amount of concussion associated with these barrels is also significant. I remember standing a few feet behind a friend of mine who was shooting a 10.5" AR-15 and I could feel the concussion of each shot against my chest. Even with a decent flash hider, the amount of muzzle flash would definitely blind the shooter in the dark.

When shooting a short barreled full-auto it's very difficult to deal with the increased recoil. Shots are often kept to short 2 or 3 round bursts otherwise the weapon tends to climb for the sky. One way of dealing with the concussion problem on a short barrel is to add a suppressor. Although, some manufacturers will not warranty their products on barrels shorter than "X" inches. Also, a short barrel means less volume for expanding gasses to bleed off. The extra back pressure from a suppressor will force a significant amount of gas out the breech making it very unpleasant for the shooter. One good way to combat this is to use a piston-driven system or adjustable system like the Noveske switchblock.