A simple tool for installing the pivot pin detent on an AR-15. The pivot pin is sometimes referred to as the front takedown pin. In addition to this tool you will need something to press down the detent, like a small punch, drill bit or flattened nail.
The "flat" version does not need any supports, the "no flat" version will need a little support. I printed with a 8mm brim but that is probably not needed on a well calibrated machine.
The hole for the pin will probably need to be drilled out to 2.5mm. A 3/32" drill works too.