Thanks for the pics! nice ideas for shooters.
Now the rivets you should look at in copper are brake shoe rivets,
there made to be set with a punch and are hollow ended Plenty strong
enough for the application, also with a countersunk head. As your
rivetting through leather as well, with a copper backing washer, that
brake shoe type is the one to try. Brass will be much to hard where
leather is involved.
Theres something wrong with the G s/ware so im writing this as an email.
Confirm you get it.