When shopping  for a bolt carrier group for your ar15 rifle, make sure you purchase the correct one. In this picture , you can see the different types of BCG that are offered.  Now all of these BCG's  are 5.56mm. But if you look at the Carriers, that is what makes them different.

    The main difference between all of these carriers is Mainly ,Semi Auto or Full Auto.

The Normal Ar15 Carrier is a Semi Auto Only Carrier.  If you look to the opposite side of the bolt, on the bottom of the carrier. You will notice there is only a little Material on the bottom.  This is so if you did put this in a Full Auto Ar15, There is No bottom, or should I say the Bottom Shelf is not Long enough to Activate the Full Auto Sear.

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