Yes, it headspaces off the rim, BUT only for the "first shot"!!!! If you push the shoulder back when reloading and allow the case to headspace off the rim for each shot, soon you will have an insipent ring inside the case and seperation that comes with it!!!

If you want any "shouldered" case to last while reloading, you have to let it headspace off the shoulder!!! It doesn't matter if it's rimmed, rimless, or even belted!!!!

Drilling Man