I had this rifle several years before I decided to get serious with it. The first time I formed a case and saw the almost non-existent shoulder I put it right back in the safe!

I did not trim before fire forming since no bullet was used. It is pretty obvious in looking at the fire formed case where the chamber ends. As pointed out by Huvius above, I probably could have trimmed the cases just a tiny bit more to clean up the mouth where it starts to roll in at the end of the chamber. My trim length turned out to be 2.207 inches.

I hope you take the plunge and get your rifles going. Is your #539 like my #352? Seems like most of these rifles that have appeared before are fancier, engraving, ribs etc. Post some pictures!!
I wish we had more historical information about how this caliber came about, how much it was used etc.
I don't think I would want to fire generic factory ammo in my rifle. Fire forming to each individual chamber seems like the way to go given the case shape.