This gentleman I work with has asked me to find him an upper for his Colt AR. Since basically every part I've ever purchased for my AR's have came from this sight, he delegated the task to me. He is a retired Georgia State Trooper of over 30 years and says he has an M16 A1 which I guess is possible with his background. Any who, he's wanting me to find him a complete upper in A2 configuration so he'll have both adjustments on his sights. He says his Colt lower has the, I guess you would call it "Old Style" front take down pin that screws in, non-detent type. The only AR's I've ever dealt with are of the detent pushpin take down variety. I know they make an offset pin but I get the impression he's looking to keep his original front pin. Excuse my ignorance but when looking for an upper on the EE what should I be looking for as far as description? He's out of town right now and when he gets back I'm going to have to find out what he wants as far as barrel specs go, 16", 20", carbine, midlength, rifle etc. It's been my experience the best ways to learn something is to ask questions, read about it, and do it. Thanks in advance for any replies.

it's called a big-pin or large hole, or "SP-1" upper, and it's got a .315" pivot pin.

You can use a military or modern .250" pivot pin upper with an SP1 style lower by using an offset adapter pin like this:

First post.

I recommend the adapter pin.

There are early Colt Sporter models that have the screw in take down pin but they are milspec .250".  I think I've seen them on Sporter Lightweights.  The lower usually has the area for the spring and detent for the pivot pin not drilled.  Have him take the pin off and measure it.  If it's .250", it's milspec and if it's drilled for a spring/detent, then it's an easy fix and he can use any milspec upper.  If it's a large pin, .315" he can use the adapter and any milspec upper or located a large pin A2 upper.

https://www.ar15.com/forums/equipment-exchange/Colt-20-762x39-conversion-upper/159-1811459/

If he’s interested in a 762x39 upper with screw

Sounds to me like he has a SP1 Sporter (.315" pivot pin holes). If he is wanting to find an A2 upper assembly that will mount using the current large hole lower receiver, I suggest looking for a Colt AR-15 A2 Sporter II upper assembly with the A2 upper receiver. It also has a .315" pivot pin hole.

The other option, as mentioned, is to find an standard A2 upper assembly (.250" pivot pin hole) and use an adapter such as this. This allows a .250" upper to be used with a .315" lower.

If it's a legitimate registered machine-gun, then he needs an AUTO Sear cut upper receiver.

The Colt Sporters did not have this feature, which is commonly overlooked.



The AUTO Sear cutout allows the receivers to actually close on each other on a registered MG lower, and the sear to articulate when necessary.

For that reason, he'll need a Mil-Std upper and will have to use the adaptor pin for a large hole Colt lower.

Did any m16s have large front pins? I thought that was only a civilian thing.

No. The .315" pivot pins were specific to the Colt Sporters.

People commonly use "M16" to denote even an AR-15 SP1 which has been converted into a registered machinegun. While not strictly correct, in most people's minds "AR-15" implies semi-auto, while "M16" means full auto.