Perry: hope the following will be of some help.
1-- Re. Mould diameter
vis-à-vis cast bullet diameter.
If I recall correctly and from reading the Lyman mould instructions (see below link) and in various editions of the "Lyman Cast Bullet Handbook", the Lyman bullet mould diameters are based on casting with the "Lyman #2 Alloy."
http://www.lymanproducts.com/lyman/bull ... gGuide.pdf
(As an aside, the "Lyman Cast Bullet Handbook), 4 th. edition, is available from Buffalo Arms for approx. $25.00, plus shipping and handling. It is well worth it as it explains the casting procedure and has loads for various calibres with
various manufacturers' moulds.)
However, too hot mould and /or too hot alloy effect cast bullet diameter. In essence, the hotter the mould becomes-- esp. steel moulds --the more it will expand. That is why pre heating the mould, casting cadence, and monitoring the melt temperature are important. Also, the alloy mixture/content will cast a bullet of different diameters s well as weights.
It takes some “trial and error” to determine what alloy, casting at what temperature, will give the desired diameter (or as close to as possible). That way the bullet will require as little sizing as possible. And the more sizing required, the more chance of bumping up the bullet nose to a larger diameter, which can/will cause leading and chambering issues.
2-- I have no experience with shooting hard cast bullets at high velocities. Have "heard" and read that cast bullets can be heat treated followed by a water quench to harden them further to prevent bore leading at high velocity. Never tried it as I do not want to use the kitchen oven to attempt heat treating lead bullets.
I have another "hunch" that after a certain velocity, the cast bullet will need a gas cheque to prevent leading. When I shot 500 g cast bullets in my Browning 1886, I loaded a gas cheque designed bullet.
3-- Veral Smith of Lead Bullet Technology authored a pamphlet about high velocity cast bullets "Jacketed Performance with cast Bullet" See below link. It might be helpful for you or others.
http://lbtmoulds.com/moulds.shtml
It might be advisable to once again call JES Rebore and ask them for a mould and alloy recommendation. Just might save you a great deal of time and frustration.
And something else to remember, since the original .38-55 Ballard-Winchester was a black powder cartridge, I have been told that there are
no SAAMI specifications for the chamber, bore and bullet size. This is why there is a plethora (pardon me while I wax eloquent) of chamber and bore sizes. And this is not unique to only the .38-55 cartridge.
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