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  • Ken Kraushaar

Did you know?..(#4)

Hopefully everyone had a safe and sane holiday week this past 4th.

Today's "did you know?" has to do with the differences between an AR-15 and an M-16.

First it is important to know the definition of a machine gun:

There are actually 4 definitions of a machine-gun if you interpret the legal-speak:

1) a firearm that fires more than one shot by single function of the trigger

2) the receiver of any machine gun

3) any device, part, gadget that has the sole function of converting a semi auto into a machine gun

4) any combination of parts from which a machine gun can be assembled.

Definition 26 USC 5845

#1 fires more than one shot per single function of the trigger.

Examples of firearms which are not machine guns would be an Arsenal AF2011-a1 double barreled 1911, or a 4 barreled COP inc Derringer, even though they would seem to be by this definition. however, in the case of the 1911, it would release one sear per barrel, thereby creating one function per barrel, or two separate functions and the ATFs legal counsel has determined it is not a machine gun. in the case of the derringer with 4 barrels, or the firing pin rotate per triggerpull, thereby firing one barrel at a time.

Fire on release.

interesting enough, conversions that fire once on the trigger pull and once on the release have been ruled by the ATF to not be machine guns.

in 2009 ATFs firearm technology branch reaffirmed that ATF counsel had earlier dertimed that as long as the gun fires only one shot on the trigger pull and one on the release that it is not a machine gun. the atf counsel approved interpretation of single function of the trigger is a single movement of the trigger, making systems like this

Definition 2": the frame/receiver of any machine gun.

example. the only difference between the receiver of an M16 and that of an AR15 is that the M16 has a pin hole for the Auto sear, whilst the AR15 does not.

It's important not to mistake colt blue box rifles for machine guns. blue box rifles were built with a hardened block pinned into the receiver to prevent the installation of conversion devices.

the pin in this case is much larger and offset, whereas the pin for the m16 was centered above the fire selector switch. and much smaller.

#3 any gadget solely intended to convert a semi-auto into a full auto.


a drop in auto sear. this item is a machine gun all by itself. it serves the same purpose as the military auto sear in an m16 however it doesn't require the tell tale hole in order to be held in place.

another example is a lightning link or auto connector.

#4 a combination of parts from which a machine gun can be assembled.


a Sten MK II parts kit which has a pipe which will be turned into the receiver if the paper template is used to cut out the proper holes in order to assemble the parts.

the ATF has ruled that the parts kit, the piece of pipe, and the template, fall into definition #4

So what is the difference between the two firearms?

The M-16 is a firearm which originated during the Vietnam war as a light weight assault weapon. It is capable of select fire, meaning it can fire in both semi-automatic and fully-automatic fire modes, and fires an intermediate round, While the AR-15 is the civilian marketed version which had to have several modifications made to it before it could be sold to the general public as a sporting firearm.

The reason for the confusion is due to the original name of the M-16 which was originally stamped as "Colt AR15 model M16-A1" on the receiver. AR-15 is the designation of the rifle that Colt purchased from Armalite, which is actually what the AR stands for in AR-15.

so what are the actual differences between the two firearms?

There are 6 parts that had to be modified or eliminated in order for the AR15 to be sold on the civilian market. the auto sear had to be eliminated completely. notice the picture provided by the IFSA, can you see the differences?

image care of IFSA

the parts which were modified are:

1) the selector switch

2) the disconnector

3) the trigger

4) the hammer

5) the bolt carrier.

also missing from the AR-15's lower receiver is the cam pin hole for the auto sear, which was a pin that was located just above the selector switch.

it is important to note: none of the m16 parts are illegal to possess under federal law, as they are legitimate replacement parts for registered machine guns.

use of m16 parts:

You will see it written in various publications that the installation of any M16 parts in an AR15 is a felony.

this is not necessarily true...

  • fact: the installation of any number of M16 parts in an AR15, which result in the ability to fire more than one shot per function of the trigger is a felony because the gun is now a machine gun (definition1)

  • therefore, possession of an AR15 and some M16 parts could result in prosecution for possession of a machine gun under Definition4

  • in some cases, the installation of any number of m16 parts can allow full auto fire due to a stack up on tolerances (too many loose parts)

  • these are ruled upon by the ATF on a case by case basis.

In order to make the 5 full auto parts work in most cases, the auto sear must be installed.

however that leaves a permanent indicator- a hole in the receiver.

ATF has ruled that the presence of the axle pin hole through the receiver, is the feature which makes an AR type receiver a machine gun (definition2)

note: it does not matter whether any other modification has been made, or whether any parts have been installed.

sometimes its all about the look.

notice that there are some AR type receivers indicating a full auto position, even though none of them have the axle pin hole. This is purely a marketing ploy by manufacturers who are selling to people who want to have the "look," however, the parts contained within are all semi-automatic and not capable of firing full-auto, despite the "look" of the firearm.

Of course they could have a DIAS and factory M16 parts installed to facilitate full auto fire, in the great majority of cases they only bear this 3rd mark for looks.

drop in auto sear (DIAS)

then someone invented a drop in auto sear that didnt require an axle pin hole to hold it in place.


1) its a machine gun all by itself (def3)

2) DIAS manufactured before 11/1/81 used to be considered legal to own, but no longer, due to a recent court ruling

3) DIAS will only work if the 5 M16 parts have been reinstalled.

the price of an registered DIAS is $8100

lightning link aka- "auto connector"

it is classified as machine gun by the ATF. (def3)

  • this device does not required that any m16 parts be used in the rifle

  • any lightning link is required to be registered.

  • current price of a transferrable one is over $9000


While they may share similar characteristics, and similar looks, the AR-15 is not military grade, and is not, from a technical stand point, the same gun sold to and used by our military. In many cases does not have the same tolerances used in the M-16. While they can be made to accept M-16 parts, this doesn't mean that they are the same gun.

Again, please note that machine guns are the one category of NFA firearm which may not be legally manufactured, except for sale to government agencies. This has been the case since 5/19/86. There are no companies out there which are manufacturing M-16s for sale to the public.

As always, educate yourself, happy and safe shooting!

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