thinning the rims is a boring work as I say, without a lathe more or less impossible. you can insert the hornet brass into a box column drill and thin the rim with a file but less fun than the other method.
the trim die is simply a sizing die with the lenght of the brass, hardened so you can cut the front end of the hornet case and hone it done with a file. the neck of hornet brass have to be open up to .250 or a little bit more so it form the right way in this sizing die.