The Geissele Automatics Trigger Fitting Pin is one of those tools that I could live without but I am glad I don’t have to. I have several decent sets of punches but I still fin myself reaching for the Trigger Fitting Pin frequently.
Most punches have squared edges on the pushing end and a barrel shaped handle. They are mostly designed for use with a hammer to drive pins. The Trigger Fitting Pin is different in that it has a chamfered end which helps it fit and align easily. It has a plastic ball grip rather than a barrel shaped grip. The ball grip allows you to hold it a number of ways but the magic happens when you palm the grip to really put some force into pushing pins.
The Trigger Fitting Pin is sized to fit AR-15 trigger pins precisely. It can be used to push the hammer pin past a stubborn j-spring or as a slave pin for when you are assembling your fire control group. I also use it to push pins on Glock, 1911s, 10-22s, and more. It is sturdy enough to hold up to a light tap or two from a mallet but I have generally avoided anything more than that to prevent the end from mushrooming.
This sits in the top compartment of my tool cart because I am constantly reaching for it. Check out the Geissele Automatics Trigger Fitting Pin.