What I found for shooting groups with 45acp is that if I re sized the loaded ammo, it pinched the .452" bullet down to a much smaller size inside the case mouth.
The Lee Carbide sizer has a carbide ring with an inside diameter of .467".
Brass or cartridges will come out ~.469".
So I reamed the chamber with a .469" reamer.
I then throated with a 45acp throater until the bolt just closed on a 230 gr loaded cartridge at 1.275".
Through sizing the base of the cartridge is another issue.
The carbide ring in my RCBS 357 mag will pass a .371" pin gauge.
357 max as well as 357 mag have ~.009" wall thickness at the neck.
If the brass come out at .373", then the ID of the brass is .355".
The lead cannot spring back a full .002" like the brass can.
When I lengthen a 357 mag revolver chamber, I use a .380" straight fluted reamer. But what can I hit with a revolver? I am great shot with a scoped rifle.
The rear of your chamber is probably .381" and the base of the your 357 max brass is probably .376", so you already have .005" clearance in the rear and no way to fix it. The way to gain accuracy is how small the additional reaming can be done.
If I had a handi rifle barrel in 357 mag that I wanted to convert, I would probably be in the neighborhood of a .373" reamer. They cost ~ $23 and come in .001" increments.
http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1704924&PMT4NO=81515581
The exact size chosen would depend on the loaded ammo I produced and used as the headspace gauge.
I would bet money that any gunsmith you can find in the phone book is going to want to chamber it so that factory ammo will fit. .380" is my guess.
There are guys that can compete with me in straight wall Schutzen rifles. They seat the bullet when chambering, and shoot cast bullets. That way the bullet enters the bore concentricly.
But with a 357 max rifle, I think you want to go faster than cast velocities, or you would stay at 357 mag.
If you do go to a gunsmith and let him have his .380" way with it, you can optimize with handloads by seating long and jamming into the lands. Pulling an unfired round could get a bullet stuck and powder spilled, but it is easy enough to have a cleaning rod around.
