AR15 Hammer Install Tool

Slide the ends of the hammer spring into the holes at the bottom of the legs, Push the top of the hammer up under top ledge so it's held in place.  Hold the unit and manipulate it so the hole lines up with the hammer pin holes in the receiver. insert the pin, when the pin is locked in place lift up on the tool enough to free the hammer which should be resting against the receiver. When the hammer is free slide the unit back to free the legs of the hammer spring.

There's a good bit of tension on the spring. I suggest wearing safety glasses and I printed this in PETG with the short side on the build plate and supports.