This article is intended to help you to distinguish between semi-auto (we'll call these AR-15) parts and those designed for full-auto rifles (referred to here as M16 parts). The BATF has interpreted the law such that parts which would convert a firearm into an NFA firearm are subject to registration including:

• Any combination of parts designed and intended for use in converting weapons into machineguns;
  
• Any part designed and intended solely and exclusively for converting a weapon into a machinegun;
  
• Any combination of parts from which machineguns can be assembled if the parts are in the possession or under the control of a person;
  

  (see ATF FAQ for more details)

Due mostly to ignorance on the part of dealers and the general public, there are many rifles out there that are violating the law unknowingly. Most will have a couple of parts, and though the rifle will not be capable of full-auto fire, will still be in violation of the law. It is important to check your rifles and those you consider purchasing, since the penalties can be harsh:

From: [49 U.S.C. 781-788, 26 U.S.C. 5861, 26 U.S.C. 5872]

Violators may be fined not more than $250,000, and imprisoned not more than 10 years, or both. In addition, any vessel, vehicle or aircraft used to transport, conceal or possess an unregistered NFA firearm is subject to seizure and forfeiture, as is the weapon itself.

Needless to say, the risks are not worth it so it is important to be able to identify the AR-15 vs.. M16 parts; included here are the trigger, disconnector, hammer, selector, bolt carrier, and auto sear. While it is unlikely that you will ever find the auto sear in a non NFA weapon, any combination of the others is possible.

The AR-15 and M16 parts are very similar, and unless you know what you're looking for, would most likely not notice. Many of the AR-15 parts have been made by modifying the plentiful M16 parts. Colt has made a number of modifications over the years to their AR-15 lower receivers to minimize the possibility of "misplaced" M16 parts, but on most non-Colt rifles, M16 parts will fit in place of the semi-auto parts. Let's look at these in detail:

Trigger
Triggers are very similar; only difference is that the M16 trigger has an open channel at the rear. This is to accommodate the M16 disconnector.

Disconnector
There are a couple of versions of the AR-15 disconnector. Essentially, they are the same as the M16 part without the "tail" at the rear.

Hammer
The M16 hammer has a "hook" at the upper rear extreme which is caught by the auto sear in the M16. AR-15 will not have this hook.

Selector
The M16 selector has a number of additional cut-outs and can be easily distinguished from the AR-15 version. In addition, the M16 selector will be capable of being rotated to the third, auto, position (pointing to the rear) while in the receiver; this is possible because of an additional index on this selector.

Bolt Carrier
The M16 bolt carrier will have about a 2" closed section at the rear that actually trips the auto sear in full-auto fire. Semi-auto versions have either a reduced closed bottom section (about 1/2"), or a completely open bottom (made so that an adapter to M-16 configuration cannot be installed).

Another difference in the bolt carriers is the additional metal removed on the underside to expose the collar of the firing pin. This is intended to catch the hammer and prevent firing if the carrier is not fully forward when the hammer releases. This occurs if the hammer is "riding" the carrier back (i.e. no auto sear is holding back the hammer). On the M16 bolt carriers, the firing pin collar is not exposed.

There are some "hybrid" carriers that do not have this area open and yet only have the 1/2" closed bottom section in the back. There are others that appear to be M16 carriers, but apparently have had 1/4" of the rear "sear trip" section milled, and so do not qualify as M16 carriers because they cannot trip a sear.

Conclusion
As mentioned previously, having M16 parts in you AR does not mean that it will be capable of automatic fire, but it does mean that you could be in violation of the law. The penalties are harsh, and definitely not worth the risks. Check those rifles, and replace any parts that don't belong. Keep in mind that some parts like the bolt carrier, can appear to be M16 and yet actually be legal AR-15 parts... stay legal, and most importantly, stay safe!