The AN-94 in action. Perhaps the rarest military rifle in the world (and certainly in the United States - evidently there is one owned by the government, and this one - only two, no more).
It's both gas and recoil operated - it has essentially a second receiver inside that uses a cable and pulley system to enable a superfast two-round burst to increase hit probability and (potentially) armor-penetration.
Ian McCollum tears a (factory deactivated) sample down. If he's calling multiple parts of the mechanism a "thing" multiple times, you bet it's complicated.