AR15.Com Archives
 Select fire lower receivers VS semi auto lower receivers
Deadman573  [Member]
7/10/2011 10:26:45 AM
I was wondering if someone could tell me what the differences are on a Armalite M15 lower (or any other semi auto lower) VS a M16 lower? I know any class 3 parts or complete rifles are not legal to own in NY nor is it legal to modify a semi lower into a full auto lower. Im not going to break any laws Im just trying to learn. So before anyone tells me the difference is 10 years in prison and a $250,000 fine there's no need I already know that part of things. I don't know about other states but in NY you cant seem to buy a Colt AR stripped lower. Anyone know why?
Knife_Sniper  [Member]
7/10/2011 7:04:09 AM
A full auto lower will have a small hole above the selector.

I doubt you will ever come across one, but avoid it if you do.
EJThor  [Member]
7/10/2011 7:14:40 AM
Originally Posted By Deadman573:
I was wondering if someone could tell me what the differences are on a Armalite M15 lower (or any other semi auto lower) VS a M16 lower? I know any class 3 parts or complete rifles are not legal to own in NY nor is it legal to modify a semi lower into a full auto lower. Im not going to break any laws Im just trying to learn. So before anyone tells me the difference is 10 years in prison and a $250,000 fine there's no need I already know that part of things. I don't know about other states but in NY you cant seem to buy a Colt AR stripped lower. Anyone know why?


Are you talking one where the lower is engraved for "Auto", or one with the extra hole? Extra hole is a no-no.

As to the Colt AR receiver, if it's Pre-Ban, and says "Colt AR-15" on it, you're fine. If it's Post-Ban, it can still be a Colt lower, it just has to say something other than AR-15 on it (Match Target, Sporter, HBAR, etc).

Deadman573  [Member]
7/10/2011 7:31:03 AM
I was wondering about the one with the sear hole. Other then the sear hole is there any other differences besides markings?
fgc  [Member]
7/10/2011 8:18:27 AM
Yes there is additional machining done in the fire control pocket to allow for the auto sear. It is not just drilling the 1/8" hole.
NAM  [Team Member]
7/10/2011 10:45:14 AM
Originally Posted By fgc:
Yes there is additional machining done in the fire control pocket to allow for the auto sear. It is not just drilling the 1/8" hole.


+1. No, i will not elaborate.
jestertoo  [Team Member]
7/10/2011 11:09:35 AM
How to convert your ar15 to full auto.. I kid.. Army video explaining M16 function.
ccosby  [Team Member]
7/10/2011 11:55:39 AM
Originally Posted By Knife_Sniper:
A full auto lower will have a small hole above the selector.

I doubt you will ever come across one, but avoid it if you do.


To make it even more confusing, some of the older colts had a larger hole above the selector but offset. It is where they had a sear block that someone has removed. They are fine.
RenegadeX  [Member]
7/10/2011 3:42:51 PM
Typical S/A lower machined to Colt SP1 specs:



Factory Colt M16A1. Note how the thin wall extends much further back for sear installation:




motoguy  [Team Member]
7/10/2011 4:32:20 PM

Originally Posted By NAM:
Originally Posted By fgc:
Yes there is additional machining done in the fire control pocket to allow for the auto sear. It is not just drilling the 1/8" hole.


+1. No, i will not elaborate.

Meh. I never understood why some people feel the need to act as the "sacred keeper for all things full auto". There's not much to it. A "little machining", and a semi lower can become full auto. Then again, a "little machining" and a block of metal becomes an engine block / receiver / axle / whatever. An auto lower has an additional hole for the sear pin. The back of the FCG pocket has more machining done (thinner receiver walls) so the auto sear can fit in. That's about it. Here's a page that may answer your questions:

http://www.quarterbore.com/nfa/registeredreceiver.htm
Deadman573  [Member]
7/10/2011 6:49:50 PM
So is it safe to say an older Colt lets say a pre ban A2 blue box would have the thinner walls in the back of the receiver where a sear would go but it will not have the small sear hole drilled into it? and for some odd reason if it does have a sear hole thats a no no and I would be smart to stay away from it?
motoguy  [Team Member]
7/10/2011 7:07:13 PM

Originally Posted By Deadman573:
So is it safe to say an older Colt lets say a pre ban A2 blue box would have the thinner walls in the back of the receiver where a sear would go but it will not have the small sear hole drilled into it? and for some odd reason if it does have a sear hole thats a no no and I would be smart to stay away from it?

Unless there is paperwork with it, -any- receiver with the small sear hole is one you'll want to stay far, far away from. That hole -is- what makes a receiver a machinegun in the eyes of the BATFE.
j3_  [Team Member]
7/10/2011 9:09:16 PM
Originally Posted By Deadman573:
So is it safe to say an older Colt lets say a pre ban A2 blue box would have the thinner walls in the back of the receiver where a sear would go but it will not have the small sear hole drilled into it? and for some odd reason if it does have a sear hole thats a no no and I would be smart to stay away from it?

That is incorrect. Even older Colt ar15s have the thicker walls in the rear section.
CAR-AR-M16  [Team Member]
7/11/2011 9:49:57 AM
Some older colts had a sear block installed to prevent the installation of a DIAS. Here is a pic from Quarterbore:


A lot of folks removed these (totally legal) and some folks mistake the sear block pin hole (which is just on one side) with the auto sear hole (which goes thru both sides). They are neither the same size nor in the same location. Here is another pic from Quarterbore:
86HMMWV  [Team Member]
7/11/2011 11:03:56 AM
Also note that an ArmaLite receiver is high shelf.
Circuits  [Team Member]
7/11/2011 2:42:00 PM
Originally Posted By CAR-AR-M16:
A lot of folks removed these (totally legal) and some folks mistake the sear block pin hole (which is just on one side) with the auto sear hole (which goes thru both sides).


There have actually been at least three sear block installation schemes from Colt. There is the one, oversized pin on the right with a vertical roll pin; two oversized blind pins (right and left) with the vertical roll pin; and a type with three oversized blind pins, one right and two left.
CAR-AR-M16  [Team Member]
7/11/2011 3:09:32 PM
Originally Posted By Circuits:
Originally Posted By CAR-AR-M16:
A lot of folks removed these (totally legal) and some folks mistake the sear block pin hole (which is just on one side) with the auto sear hole (which goes thru both sides).


There have actually been at least three sear block installation schemes from Colt. There is the one, oversized pin on the right with a vertical roll pin; two oversized blind pins (right and left) with the vertical roll pin; and a type with three oversized blind pins, one right and two left.


I did not know that there were that many different types. Thanks for the info.
Predator  [Team Member]
7/13/2011 7:21:49 PM
Originally Posted By Circuits:
Originally Posted By CAR-AR-M16:
A lot of folks removed these (totally legal) and some folks mistake the sear block pin hole (which is just on one side) with the auto sear hole (which goes thru both sides).


There have actually been at least three sear block installation schemes from Colt. There is the one, oversized pin on the right with a vertical roll pin; two oversized blind pins (right and left) with the vertical roll pin; and a type with three oversized blind pins, one right and two left.


Don't forget the current "web".