GG - this is exactly what I was doing when I broke the mandrel in mine
. When I asked about this process, that's when they told me it shouldn't be necessary, and I was probably trying to put too much force through the gun. Not saying don't do it, just be careful :'(.
D4D - Sounds like you need to play with the adjustment settings on your gun a bit. To be honest, I'm not that good at getting the gun set right myself. There's about three different things that seem to affect how "deeply" I can set an insert for a given amount of squeeze, and I've just not used it enough to really work out how they interplay.
Do you have the same problem using your air tool? Or is it just for normal rivets? (my ignorance of air tools is showing here...)
I'm afraid you have shown my oversimplified process to be obviously oversimplified, and I have no good answer for you! When I next have need to use it, I'll have an experiment with my gun and see if I can work out a combination of settings that result in a deeper "set".
I refuse to be stumped by a glorified pair of pliers!
Matto