corerftech,Last year, I had been looking at wildcatting my AR 300 BO by simply running a 300BO reamer deeper into the chamber, thusly it becomes a hotter round, particularly with 125 to 130 gr bullets for hunting. One can still use 223 brass and the std mag and bolt face. 
I found it shoots up to 200 gr cast bullets very well as is.
Then, since I am also a SS rifle affictionado, I recognized the 357mag and 357maxi have the same base dia as the 223.
So, I started forming cases based on the rimmed 357 m & MX brass.
So far, I only have some  bp capacity data on the rimmed 300bo variants. I have made some cases in various configs.   
1. Run 357 maxi case all the way to the shellholder touch into my 300 BO die. This makes for a long neck for long bodied cast bullets. Capacity is about 20 grains Swiss 3f to the base of the neck. Or up to 18 or so gr D4227.
2. Run 357 maxi case into the 300 bo die until I have about a
    0.275" neck.  This forms the case into a slightly short 30x223 size.  Capacity:  30 grains Swiss 3f to the mouth, 27 gr Swiss 3f to the base of neck. The longer, number 2 case with a 190 gr Lyman 311334 seated to just cover the grease grooves feeds into the 357m martini cadet chamber just fine. 
Actually, I could picture a person chambering a 310 cadet thusly. Based on the COW listed bullet dia for the 310 ctg, one could use 8mm cast bullets in this "310 Rimmed BO"
beltfed/arnie

 
Looking at a Win LoWall, to maybe make one up.

Strongly consider the 30-30 Wesson.

Any 30-32 cal variant that can be based on existing die profiles and use existing brass that yields a quality wildcat for SS BP service is high in my book of projects. 
The fact that a sundry reamer, sundry dies and brass I have several thousand pieces of can marry with a 30 cal barrel and give me a decent little 150 grain bpcr is way way cool. 

Now it just needs to get to .323 for a 32 blackout and just about every Swedish rolling block worthy of canabalizing is game for a conversion. 

I have a new 27 inch 30 cal barrel that I have no desire to use as is (Krag)on the martini its presently attached, it's got a 6 inch full diameter shank and is an eAsy convert to this service. Would screw into a swede action nicely. Might even run some smokeless loads over the RSI and work up cast to 25k. 

Maybe it's even a breach seatable cartridge? There would be significant BP capacity increase. Maybe that's the 32 cal version! It would be straight walled, or nearly.

Anyway I think it's a huge opportunity for a fun cartridge with some flexibility and all parts are readily available. I'm kind of sick of scrounging for parent brass all the time and taking out a mortgage to do so. This would not break the bank.

Gun should be on its way back to me from JES reboring get as a .357 maximum. Have brass all prepared and ready to load. I still will need to bush the firing pin when it comes back.

The martini action should be plenty strong for the maximum. A strong action and the maximum is not to big in diameter 

Jerry

IOwnDoubles,

I have a Martini Cadet action chambered for 357 Maximum, and shoot 180g Hornady FTX, 158g plated round nose and 140g Hornady FTX.  The cartridge length does require the lever to be pulled down to full "extraction" in order to insert, but chambers smoothly.  Extraction of the empty cartridge is perfect.  NO MODIFICATIONS WERE REQUIRED ON THE BLOCK.

My best accuracy is the 140g FTX, with 16.5 grains of 2400, at around 1800 fps.  Pretty consistent 1 1/2 " groups at 100 yards and recoil is not too bad.  Good luck with the new barrel.

Any info on .357 Max is good for me, mine is close to done. 
When I get it I will let you all know how it does. 
22 1/2" barrel, a bit on the heavy side for recoil with 200-220 grn loads.
Built on one of the "thick" actions.

Dino in Reno

When the 30 Herrett got a mention I had to chime in. I have a little Martini in 357 Herrett and it is extremely accurate using the same load I ran in a 10" Contender. Power and accuracy in a little package and a heck of a lot easier to shoot. ratseye

I use the brass to size and trim for my 30-30 Wesson.  Didn't know it was hard to get ahold of.  I stocked about 500 rounds.  Do anneal your brass though.