The cut out you're referring to is not part of the original AR/M16 specs.  It was added by Colt (and then later copied by other manufacturers), along with the notch on the top of the hammer, as a down-and-dirty way to ensure that ARs couldn't be made to run full-auto by removing the disconnector, as some folks had been doing, apparently.  That these morons risked their rifles blowing up on them wasn't a concern to them either, I guess.  The notched hammer would catch on the now-exposed firing pin spool and prevent the rifle from cycling.

Anyway, it looks like a bunch of "AR" bolt carriers were made recently from M16 carriers, but without adding the deeper ramp.  All that was done was the bottom shoulder of the carrier was milled back, which is all that is *legally* required (and even that mod's necessity is questionable).

Most experienced folks prefer the unramped carriers, as they protect the firing pin better and have more mass; closer to the original design specs.  A heavier bolt carrier cycles a little slower, and generally gives better reliability.

-Troy

This should help ya:

[url]www.ar15.com/content/docs/AR15-M16Parts/[/url]

-Troy