If you want to go the air hammer route the way I do it is as follows:
I've got a big round punch, but you could use any steel rod, or my original one was a 1/2 Ratchet Extension about 6 inches long. Used my drill press to drill a rounded depression in the end of the rod/extension. Then clamp that in a vise so it stands straight up.
Take an air hammer punch and and gind the tip down so it's wider and not pointed. Then make slight depression in the end of that (to help keep it from slipping off the rivet and denting the receiver as discussed above)
Then place the rivet through the trigger guard, set the head in the depression in the top of your vice clamped rod, use the air hammer to mash the rivets flat inside the receiver.
The rivets are ugly on the inside, but stay round on the outside.