“Large Pin Lower” - AR15 to M16 conversion?

[msmp5]

I keep hearing the term “large pin lower” in reference to a pre-86 registered receiver AR15 conversion to M16. Can anyone explain exactly what that term means and what is its significance in the C3 world? Can anyone tell if the attached pics show a “large pin lower”, or not?

 

[Zygodactyl]

delete

 

[GaryNC]

The LARGE pin AR lower were made by COLT and the pins were the the hammer and trigger pins. Colt also had a SEAR block installed that would prevent converting to full auto without using a milling machine. The sear block also prevent the installation of an aftermarket drop in trigger. The large pins were 0.174" diameter instead of 0.154" used on mil spec ARs. I know this as I once owned a Colt "Match Target Competition" rifle that I purchased after the Bill Clinton AWB. I got rid of it a few years ago when I wanted a better trigger.

 

[MachIVshooter]

The LARGE pin AR lower were made by COLT and the pins were the the hammer and trigger pins. Colt also had a SEAR block installed that would prevent converting to full auto without using a milling machine. The sear block also prevent the installation of an aftermarket drop in trigger. The large pins were 0.174" diameter instead of 0.154" used on mil spec ARs. I know this as I once owned a Colt "Match Target Competition" rifle that I purchased after the Bill Clinton AWB. I got rid of it a few years ago when I wanted a better trigger.

It was the receiver mating pins as well. And they machined away much of the bolt carrier so it couldn't trip an auto sear.

The images the OP posted show the offset adapter pins to use standard milspec dimension uppers on a large pin Colt lower.

All of this was designed to make it extremely difficult to convert them to full auto at a time when manufacturers were terrified of the "readily converted" determinations that banned all open bolt semi autos and certain others.

 

[jmorris]

Colt made the pivot pins .315” on their commercial rifles and also changed the trigger group pins to .169”. The reason was so their M16 parts that are .250 and .154 wouldn’t work in a civilian rifle. They also installed sear blocks and milled the bolt carriers.

They were the only ones I know of that did it and I think they abandoned the practice in 2009 or so.



Adapter pins are available though.

 

[mavracer]

That looks like a large pin lower with a small pin upper and the adapter pin that @jmorris has pictured. Looks like the one I have at least.

 

[AlexanderA]

Colt used a variety of differences (mentioned in this thread) to try to prevent interchangeability between its civilian AR's and the military M16's. In the early 1980's, Colt's patents ran out and several competitors entered the market. These competitors made guns that were much closer to the military standards, with the obvious exception of the "third hole" for the auto sear. Since the ATF eventually came around to agreeing that the "third hole" was the key determinant of whether a gun was a machine gun, or not, the competitors' variants became the industry standard. Finally Colt yielded and made their guns like everybody else's.

A "large pin" conversion to full automatic (which could have been done prior to May 19, 1986), is less desirable than a "small pin" conversion. (Referring to registered receivers.) This should be reflected in the price.