Zinc disconnecter for the VFC M4 gas blowback carbine. You know, the little thingy that holds back the hammer until you pull the trigger where upon it *disconnects* to release the hammer.
Zinc is not as strong as steel but it is lighter which means a lighter Zinc hammer will strike with less mass which is desirable in some situations. A zinc disconnecter is needed as using a steel disconnecter would eventually wear down the a zinc hammer part.