Registered Receiver


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The M-16 and AR-15 Registered Receiver

Please Note:  This page is still in draft format!  Please come back again and I hope to have a more complete discussion here.

I have received a tremendous number of E-mails over the years from people asking me what the difference is between the AR-15 and M-16.  I have also seen this question asked  on the various Internet forums that I frequent and all too often the person receives an answer indicating that the difference is 10-years in jail and a $100,000 fine.  While I know a few people will be tempted to try to do an illegal conversion I am extremely confident that most people are looking for an honest answer to better understand this weapon system and just how it works.  It is the purpose of this page to show and describe the AR-15 and the M-16 including AR-15s that are converted to full auto which are commonly referred to as AR-15 Registered Receivers.

Following is a photograph that shows an AR-15 on the left and an M-16 on the right.  The primary differences that should be noticed include the difference in the milling of the internal portion of the M-16 receiver including a longer fire control group (FCG) cavity and deep shelves.  This additional space is needed to allow the GI auto sear to fit, allows the arm of the auto sear to interact with the selector, and provides a surface of the spring of the auto sear to contact.   The other two differences which I hope are obvious include the auto sear on the right as well as the "J" hook or spur on the M-16 hammer.  The AR-15 in this photo is actually using an M-16 hammer that had the spur removed to allow the hammer to be used with a 9mm upper in the AR-15.

Now, I am not going to post the specific measurements or advise just how an individual would convert an AR-15 to M-16 on the website as that is not the purpose of this page.  Please do not E-mail me requesting this information but if you desire additional details I do have much more information in my AR-15 / M-16 Info Resource CD which allows me to provide additional details including the laws which make doing an illegal conversion extremely undesirable.

Now that I have said that, please refer to the following drawing that I have borrowed from  This drawing shows the difference between AR-15 and M-16 parts.  Please note that an AR-15 Registered Receiver is simply an AR-15 that was built as a Title 1 Semiautomatic rifle and was later converted to Full Auto by submitting the necessary paperwork to the ATF.  Also note that it has not been possible for a civilian to apply to build a new Machinegun and do such a conversion since 1986 and this is the reason that AR-15 Registered Receivers are so expensive these days.


AR-15 and M-16 Bolt Carriers


Following are a few photos that further show the differences in the AR-15 and AR-15 Registered Receiver (M-16)

Above:  Colt AR-15 lower that has had it's sear block removed


Above:  Colt AR-15 lower with a sear block


Above:  AR-15 Registered Receiver


Above:  AR-15 Registered Receiver (Note Auto Sear hole above selector)


Above:  AR-15 Registered Receiver (Note Auto Sear hole above selector)


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This site was last updated 03/23/04