AR15 / M16 A1 Lower - Model Guide / Build Guide / FAQ
Capt Richardson's
AR15 / M16 A1 Lower - Model
Guide / Build Guide / FAQ
AR15 / M16 A1 Lower - Model Guide / Build Guide / FAQ
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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 / M16 A1 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
"Slabside" Lower
Colt Model 601
Colt Model 602
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"Partial Mag Fence" Lower
The "Partial Mag Fence" was the second in the A1 Lower Series.
It could be found on the Colt Model 602, 604, 605, 607, 608, 609, 610
"Partial Mag Fence" Lower
Colt Model 602 / US Army XM16E1
Colt Model 604 / USAF M16
Colt Model 607 Tanker Rifle
Colt Model 608 Survival Rifle
Colt Model 609 / US Army XM177E1
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"Full Mag Fence" Lower
The "Full Mag Fence" was the third in the A1 Lower Series.
It could be found on the Colt Model 603, 609, 610, 629, 630, and Later Models
"Full Mag Fence Lower"
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 both a Partial and Full Mag Fence
Lower.
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.
I am probably the last person that should be putting this together because my
fabrication skills are almost non existent, as such I will be happy to defer to
some of the other more skilled / capable fabricators for input.
What I will try to provide is some examples of what modifications and part swap
outs should be made to "cosmetically" modify an A2 Lower so that it more closely
resembles an A1 Lower.
Lower Modification
*WORD OF CAUTION* - before things get started I just wanted to
make sure to mention that attempting to modify an A2 Lower into and 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
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 "Replica
A1 Lower" is being produced.
All types or models should have the following modifications made:
1. Reprofile 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. See the picture below for an example:
2. Reprofile 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 grinded/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.
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.
For additional information on this topic / process you can also see:
A2 Lower receiver Fence removal for XM177 old school VietNam look !
A HUGE THANKS to BUZGUN and the other AR15.com members who have "pioneered" this
process and shared their experiences.
Once you have modified your Lower you will obviously need to refinish it. For
more information on that process you can check out:
Retro Refinishing?
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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
The takedown pins had a "divot" in the center of them, 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 had a hole through the center of it, 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 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 detent or marking on the right side. These
selectors would have been "smooth". See the picture below for an example:
For additional information on the various parts/components please see:
Colt's USGI M16 series variation guide edition III
Once again, A HUGE THANKS to all of the other AR15.com members who have
"pioneered" this process and shared their experiences.
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 "Civilian 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?
IMHO you basically have the 3 following options:
A. Buy an early Colt SP1 which will have a "Sladside" 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. Price can go both ways, I have gotten a
complete rifle for as low as $650, and I have seen them go as high as $1,200.
Here is an example of my 2 off brand A1 Rifles/Lowers.
Sendra / Rock Island Armory AR's
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" Lower that was sold to the Military?
NO - There are a number of differences between the 2 which include:
A. Components - the SP1 Lowers were not made with the same components that the
"Original" Colt A1 "Slabside" Lowers were. See the component swap
out/modification info above for more details.
B. Front Take Down Pin/Bolt - the SP1 Lowers were made with a "Large Diameter
Hole" for the front takedown pin. They also used a "Large Diameter Bolt" versus
a "Small Diameter" Takedown Pin. See the picture below for an example:
C. Of course the SP1 was also sold as a "Semi-Auto", and the Military was
"Select-Fire".
5. Can you "Mix-&-Match" the "Military 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" or vice-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.