AR-15 type piston systems part 1

I forgot to mention that the lightweight T-91 (Caracal 816) bolt carrier has a piston rod block that is taller than the basic T-91 bolt carrier's. the T-91 uses a upper receiver that is slight taller than a standard AR-15, but not as tall as the Caracal 816 or HK416. The basic T-91 bolt carrier and the Caracal 816 one will both fit in a standard AR-15 upper with charging handle, although the center of the divot on the 816 carrier will not line up with gas tube hole in the front of the upper.