Here is some info to give you an idea on how the trigger group works on the open bolt LMG set up in the M16.
As you can see in the diagram, there is no typical sear surface on the hammer like you would see on a M16/AR15. The hammer becomes the sear for the bolt carrier, hence it holds the bolt carrier in the rear position until the trigger is pulled. The hammer is then held back until the bolt carrier completely closes and is tripped by the bolt carrier. The open bolt carrier has a sear notch on the bottom of it. Putting a standard carrier in a gun with open bolt trigger group will yield a run away gun. Also using the open bolt carrier in a gun with standard M16 trigger group will lock the gun up solid and you will have to remove the take down pins and wiggle it apart (seen one guy do this). There is no forward assist on the LMG since when the bolt closes the gun fires and it would be really dangerous to be push the carrier home with the forward assist when the gun fired and cycled.
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