Please note that I am not a lawyer and this is not legal advice. I’m just providing a copy as a service to the firearms community.
Here is the 1999 letter that you should be aware of – I made the two sections that I think semi-auto non-NFA AR owners should know about in bold italics:
DEPARTMENT OF THE TREASURY
BUREAU OF ALCOHOL, TOBACCO AND FIREARMS
WASHINGTON, DC 20226
MAR 29 2000
Dear Mr. :
This refers to your letter of January 22, 1999, requesting
information on the legality of possessing a registered full auto
AR15 and also possessing one or more semiautomatic pre-1994
assembled AR15 rifles. You appended a number of specific questions
relating to this subject which will be answered in the order
1. Is it legal to own both?
There are no provisions under the Gun Control Act of 1968 (GCA) or
the National Firearms Act (NFA) that prevent an individual from
possessing an AR15 registered machinegun and one or more
semiautomatic AR15 rifles at the same time.
2. If legal to own both, which spare parts for the registered gun
can you also own?
Any weapon which shoots automatically more than one shot, without
manual reloading, by a single function of the trigger meets the
definition of a machinegun in section 5845(b) of the NFA. An AR15
rifle which is assembled with certain M16 machinegun fire control
components, and which is capable of shooting automatically is a
machinegun as defined.
The definition of a machinegun in section 5845(b) also includes any
combination of parts from which a machinegun can be assembled if
such parts are in the possession or under the control of a person.
Thus, an AR15 rifle possessed with separate M16 machinegun
components can meet the definition of a machinegun, if the rifle
shoots automatically when the components are installed.
The fact that a person lawfully possesses a registered NFA firearm
does not grant authorization to possess additional non-registered
firearms. A person who possesses a registered M16 machinegun and
a semiautomatic AR15 and a separate quantity of M16 machinegun
components could be in possession of two machineguns.
We advise any person who possesses an AR15 rifle not to possess M16
fire control components (trigger, hammer, disconnector, selector,
and bolt carrier). If a person possessed only the M16 machinegun
and spare M16 fire control components for that machinegun, the
person would possess only one machinegun.
3. Is it legal to use the upper receiver off of the semi-auto AR’s
on the registered AR if they are different lengths and calibers
than listed on the Form 4’s?
Before changing the caliber of a registered machinegun you should
notify the NFA Branch in writing of the proposed change.
4. Can you have several short barrel uppers (less than 16 inches)
for the registered AR and still own semi-auto AR’s?
The definition of a firearm in section 5845 of the NFA includes a
rifle having a barrel or barrels of less than 16 inches in length.
An individual possessing more than one short (less than 16 inches)
barreled upper receiver for a registered AR15 machinegun along with
one or more semiautomatic AR15 rifles would have under their
possession of control an unregistered short barreled rifle, a
violation of the NFA.
5. If you change the barrel length or caliber do I need to notify
your office if the change is not a permanent one?
This question was answered under Question 3.
We trust that the foregoing has been responsive to your inquiry.
If we can be of any further assistance, please contact us.
Edward M. Owen, Jr.
Chief, Firearms Technology Branch
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Sources For AR Parts
The following are all vendors of AR parts including barrels, handguards, triggers, magaziness and what have you that I use and recommend:
Beware no-name knock off websites selling generic import stuff. Some of the parts are counterfeit and not rated for firearms use.