Complex sentence: The later version installed a detent and machined hole in the trigger group frame to prevent an action lock; the detent would prevent the hammer from engaging when the trigger was depressed and would prevent an action lock from occurring.
Split into simpler sentences: The later version installed a detent and machined hole in the trigger group frame to prevent an action lock. The detent would prevent the hammer from engaging when the trigger was depressed and would prevent an action lock from occurring.