So a while back I was successful in stubbing an 8mm rolling block with a mint bore so that it lined itself invisibly and gave me a new chamber to cut.

I have a few more barrels and rifles to modify same way but I did it so that I could used the rifle chambered for a pseudo pistol cartridge.

My thinking was 8mm/357 or 8mm/38 special. Hypothetically I could alter a 357 max reamer and have three steps of capacity and use a combination of dies to reload if sizing is needed. The 32 RKS gave me new options but its a breach seat. I need fixed ammo for either a loafing bunny/squirrel gun or a slamming blow the rabbit to smithereens. Of course I must fit the works into a bore that will prefer a .3235-.325 bullet in cast.

Ive been beating this dead horse in my sleep for a couple of years but finally found the right combination of generic dies that will case form and offer fodder for a new chamber.

I started with 38sp, FL in 223 die. I also did a 357 as well. one offers a tighter case mouth sooner obviously. After that I used a 32-20 FL and created a neck that was still oversize but at least reminiscent of a neck. I then used a 32 S&W L Fl die to make the undersize neck (.314 ID), then used a common 8mm expander ball to open back to .320. Obviously its imperfect and my necks are not absolutely concentric but they are undersized to the "potential" chamber and COW fireform "SHOULD" make great cases! The idea is to give me a pistol cartridge as straight walled as possible, that shoots .323 nominal cast at about 120gr and at just about any velocity I would like, with ability to be retreated and maybe bumped to 150gr as it pans out. Its simply a way to make use of an entire 8mm roller with only machining the existing parts. No metal added, no glue, no new sights, etc. Turn a pumpkin into a pie, as natural as possible. Twist is 1-9.5 and will stabilize anything I give it and probably way too much for short bullets. THATS OK. I DONT WANT TO GO ABOVE 1300 FPS and in all reality not above 1100, else that bunny will blow up!

Im reaching out to get some advice or opinions on a neck length to stay at (9/10 of one caliber per the brass I formed today). Also any input as to the 223 taper combined with a real neck albeit it very slight bottle neck.  continued

38 on top, 357 below (pics)

Exactly.
I have a special neck/throat for the 32-20 to open the case to .324.
Used it for Martini Cadet, PTG cut it for me.
I really DONT want another consumer of thin 32-20 brass

Id prefer to have sundry, avail at the range floor, 38/357 and as well if the reamer is cut correct I can make a std, long, x-long form the three parent cases, all same reamer.

Way more gusto behind the pistol brass if desired (deviate from design)

As for the stability, your over my head on the numbers.

You have a very long bullet stable at 11 twist going 1400 (IIRC)
Your saying a bullst less than half the length at slightly slower el but faster twist will or will not have a stability issue?
I have considered the 38 special, nearly the same length bullet, is stable at that vel with a twist in the high teens and low 20's. I think your saying IM GOOD TO GO!

Im no good with the twist formulas, have spent zero time doing any calculations.

Please elaborate if you will or confirm my last thought.

This (attached photo) is .33, but it's the same idea. I made the forming die with two drill bits, one body diameter and one neck diameter. I polished it a hair afterwards. If I were to make a neck size die, I'd drill the body portion .050" deeper and the neck a bit smaller. I do dies like this out of .750' pre-hardened 4140 because I have a bunch of it and use them with an arbor press.