I tried in the exampleN03 (interactive) to change the gun-position (for example, I typed '/gun/position 700 700 700 cm', but when starting some particles I cannot see any different. It seems that the starting position didn't change. So what can I do?
The gun position in example N03 is either fixed or set to be random, see ExN03PrimaryGeneratorAction::GeneratePrimaries(). You should modify that method if you want to fix the primary position by yourself. However, the /gun/position command in this case should be overridden in the PrimaryGeneratorMessenger and deactivated...