There is often a lot of and misinformation and confusion about the relationship between AR-15s and M16s. This comes from a combination of politically charged rhetoric and classic misunderstandings.

So we are going to go over the origins of the term “AR-15”, why it is different from an M16, and why they sometimes get confused for each other..

AR-15 Origins and Trademarks

The primary reason why modern AR-15s and the M16 get confused is that they both are derived from an original project developed by Armalite in 1957 and released in 1958. This Armalite-Rifle-15 was originally designed to replace the M14 in military service.

After numerous ups and downs in the design and adoption process, the US Military would decide to officially integrate Armalite’s rifle as the primary issued rifle of the infantry. However, Armalite was a small research and development firm. They did not have the capacity to produce the thousands of rifles the Military would need.

So in 1959, Armalite decided to solve two problems at once. Armalite sold the trademarks and rights of the AR-15 to Colt. This handled their manufacturing issues in addition to some of their financial difficulties.

Colt would modify the design to make it easier to mass manufacture and begin their sales of the rifle that year. As time went on, the US military would have several modifications added to the design which eventually became the M16 with Colt developing a semi-automatic only version for the law-enforcement and civilian markets.

Eventually the original AR-15 patents would expire in 1977, making it legal for other manufacturers to produce their own variants of the design. Colt would continue to hold the trademark for the term AR-15 and would be the only company able to actually label the rifle as an AR-15 before eventually ceasing production in 2019.

M16 and AR-15 Differences

While both the M16 and current civilian AR-15 come from a shared source, that doesn’t illustrate why they are different. So here are the major areas that separate the M16 and the AR-15

Semi vs Full-Auto

The largest difference is that the M16 is a select-fire rifle. This means it is capable of semi automatic fire and fully automatic fire/burst fire depending on its variant. The AR-15 on the other hand is semi-automatic only.

While you can legally get a fully automatic AR-15, this currently requires a tax stamp from the ATF and additional parts/modifications. Your off-the-shelf AR-15 will only be able to fire in semi-automatic.

Gas System/ Buffer System

The next major difference between the M16 and the AR-15 will be the gas and buffer systems. The M16 features a full length (rifle) gas system and a rifle buffer system. These systems were optimized for the M16 and are noticeably nicer to shoot when compared to carbine length systems.

AR-15s come in almost every conceivable gas and buffer system. However, since 2004, most AR-15s use carbine gas systems since they more closely mirror the M4A1 in design than the M16. Since then, the AR-15 has branched out into specialized buffers and gas systems.

Barrel Length

Barrel length is another difference between the M16 and the AR-15. The M16 is set to have a 20 inch barrel. This allows for optimal burn rates for 5.56 NATO and is the standard for all M16 variants.

The AR-15 comes in multiple barrel lengths going from below 10 inches (which requires a tax stamp) all the way up to 24 inch varmint barrels. The most popular versions currently are 16 inch barrels and pin and welded 14.5 inch barrels in order to comply with current ATF rifle barrel requirements.

Buttstock

The M16 has a non-collapsible buttstock. This features a trapdoor area that was traditionally used for your cleaning kit. The butt stock comes in A1 and A2 variations which changes the length of pull. But that really is it when it comes to your normal M16 buttstock options.

The AR-15 can have a similar buttstock to an M16, but very often follows the M4’s collapsible buttstock design. There are numerous collapsible options available which feature multiple length of pull positions, various cheek rests, sling mounting options, and even storage areas.

Rails/Forends

The forend of both the AR-15 and the M16 tends to be the most easily modified section. The M16 features non-free float handguards an can range from early triangular handguards on the M16A1, tubular handguards on the M16A2, and picatinny rail sections on the A2 to the A4 variations.

The AR-15 can have all these handguards and more ranging from Keymod to M-LOK. It also has the option for free-floated rails which makes changing the handguard a little problematic but provides less accuracy shifts when resting the gun on something.

Muzzle Devices

The muzzle devices differ between AR-15s and M16s. M16s are again limited due to being issued items. This means modern versions will feature the A2 flash hider while earlier models will feature the A1 or early prong flash hider, although these are mostly collectibles now.

The AR-15 has much more available. From compensators to muzzle brakes to flash hiders and various hybrid designs. Some can also be combined with suppressors (with the appropriate paperwork) adding more versatility to the platform.

Upper receivers

The Upper receivers are also different. The M16 comes in three different styles. The A1 carry handle, the A2 handle, and a flat top receiver on the A3 and A4. This allows you to have two different types of rear sight between the A1 and A2 while the flat top can mount any picatinny compatible optic.

The AR-15 can have both carry handles and flat tops, but every so often you’ll encounter a strange hybrid receiver. These will usually feature A2 peep sights and a picatinny rail section. While these are not some of the standard offerings they do exist which is just another difference between the M16 and the AR-15.

Conclusion

The AR-15 and the M16 share an operating heart, the direct gas impingement system developed by Eugene Stoner, but beyond that they are effectively two different guns. You can make an AR-15 look like an M16, but it doesn’t make that AR-15 an M16.

The parts that go around that heart differ and the applications each is used in will rarely line up. So take this knowledge and go enjoy your rifle knowing a little more about what it is and why people keep confusing them.