Curious.  Does anyone know approximately how many rounds can be fired through an AR15 before there's visible peening of cotter key that is used to retain the firing pin?  Can one judge how much an upper has been used based on examining the cotter key?  After examining all of my AR rifles, it seems that's the part that shows the most visible wear from use.

Also... how often should one replace the firing pin retaining cotter key?  I guess there's not much risk.

I never broke a cotter pin in any or my Army issue M-16's. I managed to break one in my AR-15 after less than 500 rounds. It's a crap shoot, and the cotter pin is one of the parts I keep multible spares for. I don't think it's possible to 'date' the use of an AR by the wear of one part.