Here are some pictures to illustrate how the hammer pin in a standard AR fire control group is retained.
Hammer with spring and pin installed - note the boss and coil over on each side extends all the way to the receiver walls (the pin ends will be inside the receiver walls):
Piano wire hammer pin retainer shown at bottom of hammer
Piano wire hammer pin retainer (Shown inside cut in center of pin tunnel) - wire bears against center grooove of hammer pin and keeps pin in place in receiver
I hope this helps...