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AR15/M16 A1 Lower: Model Guide/Build Guide/FAQ

AR15/M16 A1 Lower Model Guide
There were basically 3 different styles or models of the AR15/M16A1 lower. They included the "Slabside,"
"Partial Mag Fence," and "Full Mag Fence."

"Slabside" Lower
The "Slabside" was the first in the A1 lower series.
It could be found on the Colt Model 601, 602. The pivot pins on these receivers were of the non-captive type.

"Slabside" Lower

Colt Model 601

Colt Model 602

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"Partial Fence" or "Partial Magazine Fence" Lower
The "Partial Fence" type was the second in the A1 lower series. The Pivot Pins on this series were captive,
they were held in place by a detent housed on the added reinforcement rib.
This type of lower receiver could be found on the Colt Model 602, 604, 605, 607, 608, 609, 610.

"Partial Mag Fence" Lower

Colt Model 603 / XM16E1

Colt Model 604 / USAF M-16

Colt Model 605

Colt Model 607

Colt Model 608 Survival Rifle

Colt Model 609/US Army XM177E1

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"Full Fence" or "Full Magazine Fence" Lower
The Full Mag Fence was the third in the A1 lower series.
The A1-type lower receiver could be found on the Colt Model 603, 609, 610, 629, 630, and later models

"Full Fence Lower," or M16A1 lower receiver:

Colt Model 603/US Army M16A1

Colt Model 609 / US Army XM177E1

Colt Model 629 / US Army XM177E2

Colt Model 649 / USAF GAU-5A/A

Colt Model 653

*NOTES:
Some models could have been found with more than 1 style of lower, for example the Colt Model 609 was
found in the field with either a "partial" or a full mag fence A1-type lower.

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AR15/M16 A1 Lower Build Guide

Intro
Due to the fact that A1 lowers are basically non existent, one of the options for completing an A1 Retro Build is
to cosmetically modify an A2 lower and conduct a parts swap-out so that is more closely resembles an A1 lower.

The purpose of this information is to provide some examples of the modifications and parts swap-outs that can
be done to cosmetically modify a commercial A2-type lower so that it more closely resembles an A1 lower.

Lower Modification

*WORD OF CAUTION* (disclaimer): Before things get started it should be mentioned here that attempting to
modify an A2 lower into an A1 lower can result in a worthless and DANGEROUS lower if not done properly! The
main difference between the A1 and A2 lowers is that the A2 lower had the "weak areas" that were prone to
breakage or failure reinforced. This modification process will remove that added material and if done
improperly can greatly weaken the lower to a point where it may fail under use. If you have any doubts about
your "modification abilities", for the sake of safety, it may best to defer to a professional gunsmith for help.

The extent of the modifications will depend on which model or type of lower receiver is being reproduced.

All types or models should have the following modifications made:
1. Re-profile the rear of the receiver around the buffer tube/extension area.
This is basically a process of grinding off the reinforcement material that was added to strengthen the buffer
tube/extension area, and reprofiling the area to make it resemble the original A1 area. Use the pictures below
as an example:

2. Re-profile the front of the receiver below the front takedown pin.
This is also basically a process of grinding off the reinforcement material that was added to strengthen the
takedown pin area, and reprofiling the area to make it resemble the original A1 area. See the picture below for
an example:

3. Remove the "Safe" and "Fire" markings from the left side of the receiver.
This is a process of filling in the markings with either a putty/filler, or by welding a "bead" over them. Then the
area is ground/sanded smooth. See the picture below for an example:

Depending on which type or model of A1 lower is being made, the mag fence should be modified as following:

For a "Slabside Lower"
1. The full mag fence must be removed.
This is basically a process of grinding off the raised mag fence. Note that some sort of early-style pivot pin will
be required.

For a "Partial Fence Lower"
1. A portion of the mag fence around the mag release must be removed.
This is basically a process of grinding off the raised mag fence. See the picture below for example:

For a "Full Fence Lower"
1. No additional modifications are needed.

A HUGE THANKS to J. Zub aka "BUZGUN" for is his knowledge on this process.

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A2 to A1 Parts Swap-Out

There are a number of parts/components that also should be switched out or modified when converting an A2
lower to an A1 lower. Some of those parts include:

For a "Slabside Lower" used with the Colt Model 601 and 602

1. Front & Rear Takedown Pins
Some takedown pins were center-drilled; see the picture below for an example:

The original pins are almost impossible to find, so some people will drill a small divot in a current style pin.

2. Selector Switch
The selector switch was also center-drilled; see the picture below for an example:

Like the original pins, the original selector switches are almost impossible to find, so some people will drill a
small divot to simulate the hole in a current style selector.

3. Mag Release Button
The mag release button had a horizontal line pattern on it, versus the circular line pattern found on the
current ones, see the picture below for an example:

These are also almost impossible to find, so some people will modify (machine, file, etc) a current style mag
release.

4. Bolt Catch/Release
The early bolt catch/release did not have the area that sticks out at the bottom; see the picture below for an
example:

Like all other parts these are also almost impossible to find, so some people will modify (machine, file, etc) a
current style bolt catch.

For a "partial fence lower" used with the Colt Model 603, 604, 605, 607, 608, 609
The A1 partial fence lowers were transitional and could have been found with a mix of the original and current
style components. The original style front and rear takedown pins with divot and large head, selector switch
with hole, and flat end bolt catch/release could have been found on a small number of the very early models.
The majority of them would have used the current style mag release with the circular line pattern.

For a "full fence lower" used with the Colt Model 603, 609, 629, and later models
The A1 full fence lowers would have been found with the current style of components. The one exception
would have been the selector switch which would have lacked a position indicator marking on the right side.
These selectors would have been smooth. See the picture below for an example:

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FAQ

1. Does anyone currently manufacturer an A1 lower?
NO - There has been some recent talk of them being made, but currently only the "A2" and "M4" lowers are
available.

2. Is it possible to purchase a "used" A1 lower?
YES - There are a number of commercial semi-auto and military full-auto A1 lowers that can be purchased.
These can obviously be a challenge to find and they can also demand a pretty high price. For more on the
used/out of production A1 lowers, see below.

3. I want to build a Retro AR15/M16 with an A1 lower; what are my options?
You basically have the three following options:

A. Buy an early Colt SP1 which will have a slabside lower.
Pro - it is a Colt, the slabside lower is correct for some of the earlier AR15/M16 models
Con - cost, the large-hole screw used for the front takedown pin is not "technically correct."
You can occasionally find a lower for sale by itself, but you may also have to buy a complete rifle.

B. Buy an off-brand (Eagle Arms, Sendra, PWA, etc) A1 rifle/lower which will have a full fence A1 lower.
Pro - it will be an A1 lower
Con - it is not a Colt, quality may not be the best
Once again, you can occasionally find a lower for sale by itself, but you may also have to buy a complete rifle.

C. Buy an off-brand (DPMS, Rock River, Stag, etc.) A2 lower, and modify it to A1 specs.
Pro - it will be close to an A1 lower
Con - you have to be able to make the modifications and refinish it
See the information posted above for more info on the modification process.

4. Is a Colt SP1 slabside lower which was sold to the civilian market the same as the original Colt A1 "slabside"
M16 lower that was sold to the military?
NO - There are a number of differences between the two which include:
A. Components - the SP1 lowers were not made with the same components that the original Colt M16
"slabside" lowers were. See the component swap-out/modification info above for more details.
B. Front takedown pin/screw... the SP1 lowers were made with a larger diameter hole for the front takedown
pin. They also used a large diameter, double-headed screw versus a small diameter takedown pin. See the
picture below for an example:

C. Of course, the commercial SP1 was a semi-auto, and the military M16 was select-fire.

5. Can you mix & match the "milspec" A1 small-hole lowers and uppers with the civilian SP1 large hole lowers
and uppers?
YES - there are a number of companies that make adapter pins/screws that will allow you to use a small hole
lower on a large hole upper and vise-versa.

6. Is there a gunsmith who will do the A2 to A1 modification?
Basically, NO - There are individuals who have done these modifications, but currently no one has offered to
do this on a large scale public basis.