The BP and other tools crimp the outer edge of the primer pocket inward a few .000".
The tools steel base has a small upturned edge that sits just outside the pocket and when the hull is driven down over that it swedges the thin metal inward to tighten the pocket diameter.
Yes it works.
This makes the #57 primers a tight fit in the 209 pocket,,but they are actually only engaged at that edge, not the whole length of the primer pocket.
Wether that satisfys you or not, you have to decide for yourself.
There are home-made similar solutions if you search the net.
I wondered at one time if seating the #57 through a strip of plastic like from a plastic grocery bag would punch out a gasket of the plastic each time to take up the needed dia. Never tried it. To many other things to do and gave my 57's to a blank pistol user for K9 training.
Work fine in the 209 chambered blank guns.
