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.

[barryjyoung]

I convert 30-30, .219 Zipper, and a few other cases into two different .219 Donaldson Wasp cases. One of the chambers is longer than the other. This also requires reaming and turning the case neck after three dies to neck it down and push the shoulder back. I also make 6 Dasher from 6BR using a Whidden hydroforming (pain in the butt) die. I suppose I could just fireform, but at the price of components, that is no longer the simple fix it once was.