There are many opinions and versions around the Internet about the actual differences between the M16 and AR-15. While the AR-15 was derived from the M-16, there several distinct differences. The M-16 saw wide-spread use in the Vietnam War. The original design was equipped with a 20" pencil profile barrel. It also had long rectangular hand guards and was capable of semi or full-auto fire.

The M16 had a full length fixed stock, and originally used a 20-round magazine. The upper receiver featured an integrated carry handle that also housed the rear sight. Several variants of the original design surfaced with popular features such as an 18" barrel (or shorter) and an adjustable stock.

So what is different about the AR-15? Several things were changed from the original M-16 design. Some were simply cosmetic and others were significant reliability improvements.

• Bolt carrier/trigger group
• barrel length/profile
• stock/hand guard
• gas system
• sights

The bolt carrier and trigger is constructed different because the AR-15 does not have a full-auto or burst trigger group. The AR-15 BCG has less mass than an M-16 carrier to reduce felt recoil. Although, there are many different bolt carrier designs present in the AR-15 today.

A thicker barrel profile was added the AR-15 as the original pencil barrel design over-heated quickly. In addition, the length was shorted to 16" while specialized models have barrels as short as 8".

The stock was changed to a 6-position adjustable version that accommodates shooters with a shorter arms or ones wearing body armor.

The gas system has also been shorted on the AR-15 models. The 18" and 20" barreled M16s used a rifle length gas system. Meaning the gas tube extended the full length of the hand guard which was closer to the end of the muzzle. AR-15 rifles commonly use a carbine length or mid-length gas system.

AR-15 rifles ditched the integrated carry handle in favor of a picatinny flat top receiver. The flat top allows for a wider variety of sights and optics. Some AR-15s will come with removable carry handles that attach to the the flat top.

If you are after a traditional M16, your options are limited. While there are still some manufactured as traditional M-16s, the original vintage models are usually found second hand. Some of the actual M16s used in the Vietnam war have actually been re-distributed to local and state law enforcement. A small amount exist in the NFA registry and are considered "transferable" for civilian ownership.