"The top section 222 of the block 206 projects above the top of the lower receiver 28 into the bolt asselbmly cavity of the upper receiver 29.  Bolt carriers in prior art generally have a slot in their bottom such that their hammers can strike their firing pins.  Howerver, these slots do not extend along the entire length of the bolt carrier.  In fact, rear bottom sections of the bolt carriers for automatic firing rifles, such as shown in FIG 3, are needed in order to hit and operate their automatic sear.  Thus, if a bolt carrier such as that showin in FIG 3 was attempted to be installed in the rifle 10, the top section 222 of the block 206 would hit the bottom of the bolt carrier and prevent the upper receiver 29 from completely closing onto the lower receiver, hence preventing operational installationof an automatic firing bolt assembly."