The action spring is the spring that returns the bolt carrier of an AR to battery, nearly the same function it performs for all self loading firearms.
When a coil spring loses some length or a leaf spring loses some of its arch, the spring is starting to sack, or is sacking. It's a sign the spring is fatiguing and it's growing weak. If the action spring has sacked enough, the rifle will start having feed problems
Last edited by MistWolf; 04/25/16.
