Is a "Full-Auto" or "M16" Bolt Carrier Group Illegal?

The AR-15 bolt carrier group is arguably the most important moving part in the rifle. It's the core functional "thing" that's responsible for performing all the important functions of the firearm: It pulls rounds from the magazine and chambers them, it contains the hammer that strikes the primer on each round, it ejects spent casings after firing, it manages the gas system, and it mitigates felt recoil. 

But even if the BCG in your AR-15 is an M16 or "Full-Auto" unit, there's one thing it doesn't do.

The M16 BCG Doesn't Make Your AR Full-Auto

Putting an M16 or Full-Auto bolt carrier group in your AR-15 will not make your AR capable of fully automatic fire. Most bolt carriers found in AR-15s today have an M16/Full-Auto profile. Few semiautomatic AR-15 BCGs are found on the commercial market today.

What is an M16 BCG?

The M16 BCG is the bolt carrier group found in M16 and M4 rifles. Its design allows service-issued rifles to function with 3-round-burst and fully automatic fire. The rearward portion of the M16 BCG contains a longer steel sleeve. This bottom sleeve acts as a lug that interfaces with the full-auto lower parts kit.

What is a Semiauto BCG?

The semiautomatic, or "AR-15" BCG lacks the extra mass at the rear of the carrier's body. Shown above, the cut-out on the underside of the carrier reaches further back toward the bottom of the carrier. This lack of mass disallows the carrier from functioning with a full-auto parts kit.

Do both carriers function identically?

Yes. Whether you use an M16 BCG, or a semiautomatic BCG, your AR-15 will function identically. Unless you own a registered AR-type machine gun, like an M4 or M16 (which requires an expensive Type 3, "SOT" FFL license), there will be no difference in how the lower parts kit or upper receiver in your AR-15 functions. Both the gas system and buffer assembly work with either bolt carrier group in the same way.

Importantly, both carriers use the same bolts, extractors, ejectors, and firing pins, too. It is only the carrier -- the body that contains the bolt and gas key -- that is different between these two BCGs.

Are M16 BCGs legal?

Yes. Most AR-15s and barreled upper assemblies sold today come with an M16 BCG. These bolt carriers are not considered controlled components. They aren't serialized, and they aren't subjected to any sort of specific legal regulations.

Why use an M16 BCG?

There are good reasons for why nearly all semiautomatic AR-15s use M16 BCGs

First, the added mass of the M16 BCG reduces felt recoil. Many AR-15s are over-gassed -- especially if they use a carbine gas system with a 16" or longer barrel -- so the heavier BCG provides a simple fix for taming that extra gas.

Second, the added inertia of the M16 carrier can also slow the action slightly, helping to reduce the timing in over-gassed ARs to promote reliable cycling.

Why use a Semiauto BCG?

The semiautomatic BCG still provides some potential benefits, particularly if you're buildinga short-barreled rifle, or if you're interested in chambering a subsonic caliber like 9mm or 300 Blackout. 

The lower mass of the semiauto BCG can help to promote better cycling with under-gassed systems by preventing short stroking. It can also compensate for the lower energy produced by subsonic cartridges.