Reality seems to be a little different

It seems very easy for the snap to move sideways, so rather than form an end, the rivet just bends over like a nail. Also, it is forming a stubby end, rather than filling out the domed shape of the snap.
Has anyone tried annealing the copper rivets before installing them

I've tried it on a small batch and it doesn't seem to have made any difference. I don't know whether the amount of hammering required brings on the work hardening so as to negate any benefit from annealing.
I also tried heating the rivet whilst it was in the part, but this caused the part itself to bow, so that's definitely not going to work.
I may have a play with the resistance soldering machine, as that would be ideal for heating the rivet in situ without affecting the part. It should also be possible to heat rivets once the beast starts to come together.
Adrian