AR15.Com Mobile
SWFA CMMG VERIFORCE PK JT BLUE FORCE GEAR VORTEX RRA RAINIER BROWNELLS DEL-TON
STAG BRAVO AIM POF FSI DPMS MAGPUL MI SKD GALATI LARUE
» 0 Online
AR15/M16A1 Lower Receiver: Model Guide/Clone Building Guide/FAQ, redux
FITTER  [Team Member]
Originally posted by captrichardson (edited):

AR15/M16A1 Lower Receiver: Model Guide/Clone Building Guide/FAQ

AR15/M16A1 Models
There were basically 3 different styles or models of the AR15/M16A1 lower receiver. They included the "Slabside," "Partial Mag Fence," and "Full Mag Fence."

"Slabside" Lower
The Slabside was the first in the A1 lower receiver series. It is distinguished by its smooth-sided magazine well (right side).
These early receivers could be found on the Colt models 601 and 602. The pivot pins on these receivers were of the non-captive type.


Slabside Lower


Colt Model 601






Colt Model 602





__________________________________________________


"Partial Mag Fence" Lower
The "Partial Mag Fence" (or "pivot pin detent housing") -type was the second in the A1 lower series. The term "magazine fence" is actually a misnomer in this case and didn't exist per sé until later with the introduction of the true A1 series receivers; the upper portion of what would later become a magazine release fence was nevertheless present in the form of a pivot pin detent housing.
This type of lower receiver be found on the Colt models 602, 604, 605, 607, 608, 609, and 610.


"Partial Mag Fence" Lower




Close-up of the detent housing:




Colt Model 602/US Army XM16E1


Colt Model 604/USAF M16


Colt Model 605


Colt Model 607


Colt Model 608 Survival Rifle


Colt Model 609/US Army XM177E1



__________________________________________________


"Full Mag Fence" Lower
The Full Mag Fence was the third in the A1 lower series. This receiver differs from the later A2 receiver with its lack of reinforcement modifications at the rear receiver extension lug and the front pivot pin lugs. The A2 lower receiver is beyond the scope of this article.
The A1-type lower receiver could be found on the Colt Model 603, 609, 610, 629, 630, and later models. The 604 also used a full fence lower after 1967. (Thanks, scottryan)


"Full Mag 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.

For additional information on the various models and their configurations please check out:
The NEW Real Deal AR-15/M-16 retro picture thread





AR15/M16A1 Lower Receiver Clone Building Guide

Introduction
Due to the fact that A1-type lowers are basically non-existent on a commercial level, 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 it more closely resembles an A1 lower.

The purpose of this diatribe is to provide some examples of these modifications..


Lower Receiver Modification

*WORD OF CAUTION* (disclaimer): Before things get started it should be mentioned here that attempting to modify an A2-type commercial lower receiver 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 weaker areas of the earlier styles that were prone to breakage or failure reinforced. The modification process described below will remove that added material and can greatly weaken the lower to a point where it may fail under use. If you have any doubt about your mechanical skills and abilities, it may be best to defer to a professional gunsmith for help.
It should also noted that some of these modifications involve the use of welding. If done improperly, welding on an aluminum forging or casting can result in a reduction of structural integrity.
PROCEED AT YOUR OWN RISK.
The author of this thread and ARFCOM do not condone these modifications in any way, and assume no liability whatsoever for damages or injuries. These examples are presented for the sake of discussion only.

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





All types or models should have the following modifications made to be considered cosmetically correct:
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. This may include some welding or filling of some sort. 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.
As in the above example, this is basically a process of grinding off the raised mag fence. See the picture below for example:

Notice the length and shape of the rear portion of what is now the pivot pin detent housing.


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

Also, check out this thread by ar15inmn:
A2 to A1 Lower Conversion

A HUGE THANKS to BUZGUN, ar15inmn, and the other AR15.com members who have pioneered this process and shared their experiences.



Once you have modified your lower receiver you will obviously need to refinish it so it will match the finish of your upper. For more information on that process you can check out:
Retro Refinishing?


__________________________________________________

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 the Slabside Lower used with the Colt models 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 dimple 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 dimple 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. These items *MAY* become available in the near future, so stay tuned to these pages.


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. There is also a "second generation" catch which had the serrations, but also had the extended toe as on the later versions:

Thanks to scottryan for this photo (used with permission).


For a "partial fence lower" used with the Colt models 603, 604, 605, 607, 608, and 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 drilled holes 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 models 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 on the right-hand end. 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" commercial 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 - Besides the obvious differences between an M16 and a semi-auto SP1, there are a number of differences between the two which include:
A. Components - the SP1 lowers were not made with the same external 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. As mentioned above, 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?
So far, NO - There are individuals who have done these modifications, but currently no one has offered to do this on a commercial level, however, stay tuned for further developments! Remember, you saw it first on ARFCOM!



Elvis-Ar15  [Team Member]

Originally Posted By Blanco_Diablo:
This needs a tack, yo


I'll second that.

Elvis
OlSarge2K  [Member]
Wow, - Thank You.
P0WERWAGON  [Member]
what is the best way to fill in the unwanted writing, logo's?

jb weld
tig weld
mig weld
epoxy?
1234bird  [Team Member]
Question !! If you use jb weld or epoxy won't the reparkerizing of the reciever damage these type of repairs.
norseman1  [Member]
In theory, the JB weld (thinned out with a drop or two of acetone) would fill in the unwanted lettering and such.

It will NOT parkerize, though.

But, it WILL take well to Duracoat and Durabake. (I can attest to the use of DuraBake first hand)

As a follow up question, what is the reciever color for the current crop of marketplace A1 uppers floating around? Is it the classic "colt grey" or is it more of a green park?

And, what color were "most" stocks and forearms? Green, grey, or black?

(I had always assumed it was black, hence the moniker "The Black Rifle")
Blanco_Diablo  [Member]

Originally Posted By norseman1:
In theory, the JB weld (thinned out with a drop or two of acetone) would fill in the unwanted lettering and such.

It will NOT parkerize, though.

But, it WILL take
well to Duracoat and Durabake. (I can attest to the use of DuraBake first hand)

As a follow up question, what is the reciever color for the current crop of marketplace A1 uppers floating around? Is it the classic "colt grey" or is it more of a green park?

And, what color were "most" stocks and forearms? Green, grey, or black?

(I had always assumed it was black, hence the moniker "The Black Rifle")

What about Norrells?
norseman1  [Member]
What about the sear pin from an M16??

If I am using a non-nfa class 3 reciever, could I put in a "fake" sear pin on my retro build or would I be tempting fate?

It my understanding, the pin was eliminated from semi rifles since they have no need for it with the semi-only sears and components.

Since my DPMS can only use semi parts anyhow, I am wondering what folks have heard.

My other option is just to drill/tap and put a "fake" pin in that only goes an 1/8" deep. (not through)

Thoughts?

Just wondering how far into this I wanna get.
FITTER  [Team Member]

Originally Posted By norseman1:
What about the sear pin from an M16??

If I am using a non-nfa class 3 reciever, could I put in a "fake" sear pin on my retro build or would I be tempting fate?

It my understanding, the pin was eliminated from semi rifles since they have no need for it with the semi-only sears and components.

Since my DPMS can only use semi parts anyhow, I am wondering what folks have heard.

My other option is just to drill/tap and put a "fake" pin in that only goes an 1/8" deep. (not through)

Thoughts?

Just wondering how far into this I wanna get.


I think this has been done, although the hole was purposely drilled out of spec to avoid any sort of insinuation by JBT types. Another idea would be to make a special mill, like a miniature hole saw, that would cut a small circle that would represent the pin.
Sanlanman  [Member]
Great FAQ and thread. I have one of the CDNN A1 uppersand want to build a semi auto A1 looking rifle. My upper has a slight greenish cast to the dark gray finish, probably from long term storage.

Fulton Armory shows an A1 type rifle, but I can't tell from their website if they sell the reciever only.

Been away from the board for a while due to marriage, house renovations and an old car resto, but I am now getting back into firearms and particularly the AR15 type.
So I guess that makes me an old semi FNG.
APDL  [Member]
wow well done!
norseman1  [Member]
I did a search and couldn't find the EXACT answer I was looking for.

What would be the "harm" or legal woes if I added an "AUTO" or "FULL" markings to my lower?

I am going to cover up (with some JB Weld) all the markings for the mfg, and right side indicators.

But, if I ADDED the script for Full/auto, do I win a free trip to Club Fed???

BTW, I really like the idea of the fake pin, but, in a different hole location.

Still have not decided if I am going to mess with the fence or not. I think I might just do it and see how it goes.

The lower and upper will be recieving a fresh coat of "Colt Grey" duracoat here in just a bit.
Blanco_Diablo  [Member]

Originally Posted By norseman1:
I did a search and couldn't find the EXACT answer I was looking for.

What would be the "harm" or legal woes if I added an "AUTO" or "FULL" markings to my lower?

I am going to cover up (with some JB Weld) all the markings for the mfg, and right side indicators.

But, if I ADDED the script for Full/auto
, do I win a free trip to Club Fed???

BTW, I really like the idea of the fake pin, but, in a different hole location.

Still have not decided if I am going to mess with the fence or not. I think I might just do it and see how it goes.

The lower and upper will be recieving a fresh coat of "Colt Grey" duracoat here in just a bit.

Completely legal as long as it dont go bang more than once with each pull of the trigger (unless its registered of course).
WA-Tom  [Member]
A warning about welding lowers. In my limited experience you DO NOT want to weld much if at all on a lower. They tend to bow like a bannana making it difficult or impossible to close the upper on the lower. I am not the best welder but I have experienced this on about 3 or 4 lowers and it really makes me nervous when I did. I know there are people out there that weld lowers and stuff, but in my inexperience I don't see how it is done properly. I know that when it comes to filling in markings that Tom Sawyer wisely used putty as a filler instead of weld. When I discussed this with Steve he found the same difficulties that I did in welding on lowers. My $.02 worth.

Oh and good guidline! Some good tips out there. Also one of the reason so far that no one is doing reprofiling is the truly high cost and limited market.
buzgun  [Member]

Originally Posted By WA-Tom:
A warning about welding lowers. In my limited experience you DO NOT want to weld much if at all on a lower. They tend to bow like a bannana making it difficult or impossible to close the upper on the lower. I am not the best welder but I have experienced this on about 3 or 4 lowers and it really makes me nervous when I did. I know there are people out there that weld lowers and stuff, but in my inexperience I don't see how it is done properly. I know that when it comes to filling in markings that Tom Sawyer wisely used putty as a filler instead of weld. When I discussed this with Steve he found the same difficulties that I did in welding on lowers. My $.02 worth.

Oh and good guidline! Some good tips out there. Also one of the reason so far that no one is doing reprofiling is the truly high cost and limited market.


Do you weld with a modern TIG machine with high frequency start and a Amperage control?

The secret is to use copper plates inserted into the magazine and firing control cavities to act as a heat sink this keeps warping down to a minimum.
edwin247  [Team Member]

Originally Posted By buzgun:

Originally Posted By WA-Tom:
A warning about welding lowers. In my limited experience you DO NOT want to weld much if at all on a lower. They tend to bow like a bannana making it difficult or impossible to close the upper on the lower. I am not the best welder but I have experienced this on about 3 or 4 lowers and it really makes me nervous when I did. I know there are people out there that weld lowers and stuff, but in my inexperience I don't see how it is done properly. I know that when it comes to filling in markings that Tom Sawyer wisely used putty as a filler instead of weld. When I discussed this with Steve he found the same difficulties that I did in welding on lowers. My $.02 worth.

Oh and good guidline! Some good tips out there. Also one of the reason so far that no one is doing reprofiling is the truly high cost and limited market.


Do you weld with a modern TIG machine with high frequency start and a Amperage control?

The secret is to use copper plates inserted into the magazine and firing control cavities to act as a heat sink this keeps warping down to a minimum.


I was wondering the same thing. Low amp TIG welding might be the answer to just welding the roll marks. What is the perpose of copper?
Ronzo  [Member]
Tig Welding with Argon (not star gas ) will give good results, If you are fast with the rod. It is also a good idea to build a jigable to hold the interior AND exterior of the receiver. Push the arc and feed the rod from the top not the side to avoid pitting as you will have to reweld to fill the holes. OR.... Find a custom machine shoppe.
If you don't have extensive experience weld brass or aluminum, don't go there.
Hope that helps.
Ronzo.

























desertmoon  [Team Member]
Damn, guys! AWESOME thread!!!
Ronzo  [Member]
I called a freind in Ohio and he said there was a class 2 manufacturer named Charlie Erb that used to reweld demill and dewatt receivers before the 86 ban on further manufacture of machine guns. I have looked through two piles of old copies of Machine Gun News and old Firepower magazines, I'll keep looking or I'll call LMO and see if I can't find the article, It had pictures and everything.
_Anarchist_  [Member]
Just a quick note: "PWA" lowers went from the A1 config to the A2 somewhere between 17,120 & 25,000 in the Ser Numbers, just looking at the ones I have.

Folks looking to buy an A1 lower with a very minimum work needed might want to keep that in mind.

Qualitywise they are as good as any mil contractors part, the rollmarks are minimal.

Good luck
Blanco_Diablo  [Member]
HA! I just realized I have that modified DPMS in the pic. (I mean that exact lower..as in that lower with that serial number)


Guess it's something of a celebrity.
lawdawg430  [Team Member]
that about covers it. good job!
MarkRSims  [Member]
So, I am considering building using a "slabside" upper with the small pivot pin hole, and a "slabside" lower with the same size pivot pin hole (no mag fence, no place for the pivot pin detent/spring to be captured).

Is there a vendor that sells the smaller "screw type" pivot pin to fit this combination?

I have seen the ones to mate larger with smaller holes, and this is the only real stumbling block I've come across.

Mark
swgaboy  [Member]
My A1 lower came in today. Time to start to work.


Valer  [Member]
Thanks alot for a great guide.
seahorse  [Team Member]

Originally Posted By swgaboy:
My A1 lower came in today. Time to start to work.
i103.photobucket.com/albums/m148/swgaboy/P1010088.jpg
i103.photobucket.com/albums/m148/swgaboy/P1010087.jpg



swgaboy,
What would you call the color of those Tactics lower receivers? Do they have green in them or are they Colt Grey? Thanks