Hardest/most elaborate case conversion you did?

[challenger_i]

Making a rimmed version of the 6.5 TCU from 5.6x50R. 1st trick is finding the parent brass!
Anneal, FLS in a Hornady die (decapping/expanding stem removed), trim to 46mm, fire-form in a chamber insert made for 410 scatter gun (hey! I had a special chamber reamer made for this "non-standard caliber: may as well get some use out of it!), neck ream the case while seated in the sizing die (had my gun doctor make a reaming tool to fit the Hornady die: works like a champ!), Fire-form again. Load, go punch paper.
Is TRULY a labor of love, and I am pleased I have 200rnds completed, so as I NEVER have to do THAT, again!

[Good Cheer]

Recently found a long ago bunch of formed, annealed, turned .308's that I'd made out of .270 Winchester for precise necks.
Hardly a tough or elaborate conversion job but hey OK it was fun.

[Ben_51004]

Old thread, but good stuff here! I've done .40-65 from .45-70, that one's a no brainer. Slightly more challenging was .221 Fireball from .223 Rem. Pretty easy with C&H forming dies. Currently making .219 Zipper from 30-30 Win. Form #1, Form #2, file trim, FL size, expander die, final trim on Forrester trimmer, then neck turn. That's the most involved conversion I've done, but it's working.