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."
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.
Colt Model 601
Colt Model 602
"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 603, 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
"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
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.
AR15/M16 A1 Lower Build Guide
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.
*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
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
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.
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
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
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:
1. Does anyone currently manufacturer an A1 lower?
YES. - NoDak has a full line of early uppers and lowers. Including the Prototype Trigger Charging Handle
2. Where am I supposed to find gray colored parts?
A. Check GunBroker or the AR15.com EE section.
B. Jerrell Sharp (Heatnbeat) makes some very convincing reproductions. Contact him at JRS @ outdrs.net
Above is not a live link, copy it to your address line, then remove the spaces beside the "@"
C. For later non-dimpled parts you can try the "Navel Jelly" trick.
Some companies parkerize their lower parts before painting them black.
If you coat them with Navel Jelly for 30-60 seconds you will be able to wipe off the black paint leaving you
with pristine gray parts.
This works with DPMS parts and possibly others.
Rock River Arms lower parts are known to NOT work.
Navel Jelly strips them to bare steel.
Note: This only works with the steel parts.
Aluminum parts for all companies do not work.
D. You can send them to someone to be parkerized, or do it yourself.
E. Paint with either Duracoat or Norrels.
3. 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.
4. I want to build a Retro AR15/M16 with an A1 lower; what are my options?
You have the five following options:
A. Buy a NoDak Spud LLC Lower.
Pro - it will be an A1 lower
Con - it is not a Colt and will not have all the correct markings, but most of the markings can be added.
This is the most often recommended path.
B. 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.
C. 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.
D. 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.
E. Build your own from an 80% lower.
Pro -it will be close to an A1 lower
Con - even more work is required to finish
5. 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.
6. 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.
7. 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. Instead look at getting NoDak lower.
RETRO BLACK RIFLE - RETRO LOWER BUILD GUIDE
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