The 357 Maximum case is the parent case to a few BP era cartridges but, non ever had a .319 - .321 groove. The BP groove diameters were approximently .309 - .312.

What people are using in .321 barrels, are 32 Miller long, a tapered cartridge, one using a 223 body and 32 neck or the shortened (1.47) 32 RKS. It also uses the 223 body.

Frank

Hi Plumloco,

Below is a print for the reamer PTG made for my .32RKS.

I'd look carefully at the reamer that you end up with if you want to use fixed loads in the rifle.

FWIW,
One can run a 357 Max into a 300 blackout die
to whatever depth one wants and make a very nice 
"30-357" ctg case.   
And then use a 223 reamer to cut chamber body followed by a 
neck/throat cutters to finish.
OR, as you are really looking at a 32-357 max, you can still follow the route of initial case forming, neck up with .32 M die and cut neck and throat to appropriate .32 dimensions
beltfed/arnie

So how does the 300 BO compare to the 30 Carb?

plumloco wrote on Jun 13^th, 2016 at 12:18pm:


When you order your reamer, i suggest that you design it with .34" long parallel walls in the neck area, rather than continuous  taper, that will hold your bullets better and straight. Before you order that reamer, make a few dummy cases and neck turn the necks a bit to ensure uniform thickness of brass, add 0.002" or 0,003"clearance and that is how the reamer should cut. The straight parallel neck will hold the case and bullet nice and straight. after a bit of fouling. Choose a bullet to match the rifling twist, or the other way around, and then order the throat reamer with a gentle taper that matches the bullet's nose taper.  
That's what i would do.
Wait, i already have, on a .32 x 2" of sorts. 
(because you describe fitting bullets in the case).