If you own an AR-15 or you are thinking of purchasing one, you should make yourself familiar with dwell time. Dwell time is simply the amount of time that the bullet spends in the barrel after it has passed the gas port and before it exits the muzzle end of the barrel. Too much time here results in an over gassed system which can cause accelerated wear and tear. Too little dwell time can cause short stroking (especially in cold weather). Both situations are bad.
Barrel length, gas system length, and gas port size are key to determining dwell time. An understanding of these factors is especially important when you are looking at 16″ and 18″ barrels. Many people choose barrels based on looks or style without truly understanding the consequences of the choices that they are making.
The military uses carbine length gas systems (about 7″) on 14.5″ barrels and rifle length gas systems (about 12″) on 20″ barrels for a reason. Carbine gas systems on 16″ barrels may be less than ideal since they are often over gassed. Straying outside these norms can have a negative impact on your reliability. Midlength gas systems serve the purpose of shortening dwell time and over gassing on 16″ barrels. Arm yourself with as much information as you can before choosing a barrel!
Here are two excellent discussions of dwell time, its determining factors, what it might affect, and how to tune it:
How the AR-15 Direct Impingement gas system works – by Randall at AR15Barrels.com
Carbine vs. Mid-Length Gas System on a 16″ Barrel [2010-01-10] – by USMCo3 at 03 Design Group