Throwaway the cutter pen, use this on every build - Guntec USA AR-15 Firing Pin Retainer

When I discovered that Stoner himself designed the BCG with this little gem, I dug in to find out why they switched to a cotter pin. Simple answer; easy to replace when lost. Makes sense…in the field. But for the rest of us civilians, this is by far the better play. Never again will you gave to squeeze a splayed cotter pen into that tiny hole on the other side…and you know what I mean :-/