Just curious about this. The bottom rear portion of the BC where it is notched for the buffer detent.......Why do some have more metal there. Like the one I'm looking at now is 3/4". I've seen others that are less, maybe 1/2". Even seen these differences in the same brand (Armalite, RRA, etc...). Just curious as to what purpose that served. Anyone?

Are you talking about this?

Bolt Carrier

If so, the bottom one is an M16 (full-auto) and the others are semi-auto. It's heavier, I think to increase reliability and also it allows it to take the pounding that a full-auto rifle can dish out. For what its worth, they will all work in a regular AR-15.

The "longer" bottom is a M16 Carrier. The longer length is needed to trip the auto sear.
M16 Lower and Trigger Group...the auto sear is viewable along with the
pin hole for through mounting above the Safety/Fire Selector Lever. The blue part that sticks above the receiver is what the bottom of the carrier trips when the carrier moves forward with the trigger is held and the selector in auto. The shorter bottom of the SA carrier will not reach this trip point.





 

Yep, that's the part I was talking about. I just wondered why some had more meat down in that area (the semi-auto carrirers). Thanks.

More metal = Auto carrier (bottom one)

There is also the SP1 carriers that have less metal there than the semi-auto carriers. SP1 carriers are ideal for using a lightning link. And not sure who, but there are a couple I've seen where there is no metal there at all.

Just saying.