I've learned the hard way it pays to have a few extra of these around. This is the pin that holds your firing pin in place. Price is $3 shipped. This item will be shipped first class mail if ordered by itself.
179210709AR15 Firing Pin Retaining Pinhttp://cdn.shopify.com/s/files/1/0266/1345/products/AR_firing_pin_retaining_pin_large.jpg?v=1475776778https://cpwsa.com/products/ar15-firing-pin-retaining-pin3.00USDDPMSInStockAR15 Factory and Standard PartsAR15 Upper Receiver Parts