Mil-Spec AR-15 Action Spring (Recoil Spring)

If you actually shoot with any regularity, you'll get to 5000 rounds within a year or two.

Stock up on action springs (buffer/recoil spring) for each gun that gets shot.  Check your springs by comparing a new one to your spring that's in use.  If your used spring is getting shorter than a new one, swap it out.  

Weak springs cause malfunctions.  Keep your gyat running strong.