The method of operation of the open bolt fire control group is completely different than that of the M16 series of rifles and carbines. The below diagrams from the manual best illustrates how the hammer acts as the sear. The trigger actually pulls the hammer down and out of engagement with the bolt carrier notch. This allows the bolt carrier to go forward and strip and chamber a round. When the bolt carrier fully closes, the hammer is release by the auto sear to fire the cartridge. The cycle continues until the trigger is released or the magazine runs out of ammunition. When the magazine does run out of ammunition, the bolt catch is activated and the bolt carrier is locked back on the catch. When the operator pushes the magazine release button, the extension on the bolt catch is engaged by the button and the bolt carrier drops down onto the hammer/sear and is ready for firing. There is no need for manipulation of the bolt catch to bring the weapon back into action once it is reloaded.

 Contrary to the many internet myths about the wear and tear on the lower receiver caused by using the hammer as the sear, I have not experienced any significant wear in two different guns with over 30,000 rounds through them.  Most internet posters on the subject have never owned or shot the open bolt version of the Colt M16 LMG.

(Click on the image to see full size version)