i use a baking tray, stand the brass up in it and then fill it with water to the level that i want to stop the heat going past the neck and shoulder. then take a torch and heat each one until its got the faintest glow. tip it over and move on.
i havent noticed groups opening up after it. but maybe i just havent been paying attention.
Try it and if it has the same effect then we both have issues.
the other thing ive tried is holding them in my hand. that way they defiantly dont get to hot.