I know I've seen side by side comparisons of the two, but cannot find them now.  Anyone have a link or pics?

 Sorry, I don't have a link.  The only difference that I am awear of is between the selector and the rear takedown pin there is a small hole through the receiver for the shaft of the autosear pin.

Scott

The rear shelf cavity is the only difference on a Non-Colt receiver.

The M-16 has the threw-hole (third hole above the selector) for the auto sear, and the rear shelf is milled out to accept such.

On colt AR-15 receivers, the front take down pin is milled for a screw type pin (not push threw).  Regarding the Colt AR-15 rear shelves.  The SP-1 receivers were normal AR shelves (open), then came the block that was added to the shelf/receiver to prevent LL's/Dias from working in the rifles. As of current, the shelf section front of the rear take-down shelf to the back after the selector is not milled out to prevent the LL's/Dias from working in the receivers, so they no longer install the blocks.

I'm wondering because of the differences I've seen between Eagle Arms and DPMS lowers.  The EA have a higher shelf where the rear takedown pin is.  The DPMS shelf is much lower, but there is some additional thickness on the receiver wall near the back, on the right side if you are looking down into the receiver.

M16s are usually built to the same spec.

Semi-auto guns are built to all different specs, and there is not standard in how they mill the lower, as long as it cannot easily accept an M16 Sear.

[url]www.quarterbore.com[/url] has the pics you require. Great site!

And those tiny little sear pin holes which the ATFE considers a machinegun in itself in some cases, makes all the difference...

Ah, should have searched a bit harder at Quarterbore - found them this time.  AR on the left, M16 on the right (photos stolen from Quarterbore, of course [img]http://community.the-underdogs.org/smiley/happy/winkani.gif[/img]).

[img]http://www.quarterbore.com/images/nfa-ar-m16-01.jpg[/img]