The AR-15 is perhaps the most popular carbine in America.  It's commonly compared to the AK-47 despite the vast number of differences between the two weapons.  While AR-15s are modular weapons meaning many of the parts can easily be replaced or upgraded, barrel length is often left unchanged at 16 inches. There are many reasons to deviate from the standard 16" barrel length.  Many M16 rifles were built with a 20" barrel that gave it a little better accuracy.  Some AR rifles are available with 20", 18", and 14.7" barrels are also become popular.  When switch from a 16" barrel to something longer or shorter you will more than likely have to work on the gas system and/or buffer.  This is because once you start messing with barrel length it changes the amount of a pressure in the chamber.  Using different configurations of gas tube lengths or pistons and buffer weights is an effective way to control the recoil and proper cycling of the weapon.  Some manufacturers are even producing gas tubes or upper receivers that allow you to adjust gas pressure.  Gas tuning is also needed once a suppressor is attached the weapon.

Back to barrel length, the main reasons for going shorter or longer are generally weight and accuracy.  A 14.7" barrel is legal if the flash hider is permanently attached (pined and welded).  Counting the length of the flash hider it usually brings the barrel length to 16" or more.  If you plan to use a 14.7 inch barrel without welding the flash hider, you would need to submit a form 1 as it would be less than 16" and deemed a short barrel rifle.

Popular SBR (short barrel rifle) configurations are 11.5", 10.5" and even 7.5"  These configurations can shave a few pounds off the total weight on the weapon.  The only downside is the increased recoil and slightly less accuracy at longer distances.  Choosing a fluted barrel can save even more weight but usually comes with a premium price tag.  If you are considering any of these changes you probably know it won't be cheap.  Customizing an AR-15 which is normally around $1000 for a stock rile, can run well over $3,000. The following photo shows an 11.5 Adam Arms upper with a special buffer and spring.

Remember that if you go with a short barrel rifle and plan to sell it later you may be waiting a while.  You must follow all NFA rules which require the weapon to be transferred to another civilian.  This limits the amount of people who will be able or willing to purchase your weapon. The second hand market for NFA weapons is not great unless you a dealing in something fully automatic.