That's the detent spring. If you look closely at the takedown lever you will see a slot into which it fits in the closed position.
Without the spring in place, the takedown lever can freely rotate and move from locked to open. During recoil there is a mili-second when the takedown lever is not under pressure from the main spring and without the detent spring in place the take down lever can rotate and the gun will partially disassemble. Not a desirable situation.
Bottom line: the spring is essential.
John
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