https://midstatefirearms.com/product/firing-pin-retaining-pin-retainer-ar15-m16/

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AR 15 Firing Pin Retaining Pin, aka Firing Pin Retainer, aka AR 15 cotter pin, aka that forked pin that is sometimes a pain to reinstall.
Maybe you’ve lost yours. Maybe you’ve bent it. Maybe you got frustrated when reinstalling it and tried to hammer it in, and learned that hammering it in doesn’t work and destroyed it.  Regardless, it’s good to have a few spares, right?

Turn them sideways (fork horizontal) when installing, and press down a little bit when pushing it through the other side. Some take a bit of wiggling to get aligned just right when going through the other side of the carrier.