Craftsman Pop Rivet gun?`

You just catch it with your hand or let it fall on the floor.  But
until you allow it to be released from the gun, it stays in the gun.
To release the snapped off rivet mandrel you have to push in and Hold
the metal button on the side of the unit.  Then the internals jaws
that bite into the mandrel return to Start position, freeing the
snapped off rivet mandrel to be extracted.

Yes that is exactly correct.  The detached mandrel (from the rivet)
stays in the jaws of the device until you release the pressure of the
jaws and allow them to return to the Start position, wherein you can
insert a New mandrel/rivet.  The return to Start position seems to be
controlled by some kind of spring, as long as you hold in the Metal
Button on the side of the unit.

ONE MORE THING I FORGOT TO TELL YOU:

In some instances, if you use a rivet that is too long for the
materials you are attaching to eachother, you will find that the
"stroke" of the grippers jaws are not long enough to snap off the
rivet mandrel.

In that case, push release button (metal) on side of gun and allow the
gun to move FORWARD on the mandrel to get a "new grip."  Now start
squeezing/pumping the device again and 'BANG' now the mandrel snaps
off.

Lg