Quote:
Originally Posted by rwsmith
I have no idea whether these formulas work or not, but I've seem them on cast bullet forum and cast bullet manufacturers web sites.
BHN x 1,422 = Pressure needed to "upset" or "obdurate" the bullet properly.
or
BHN = ( Cartridge Pressure / 1,422 )
The 1,422 number comes from converting the pressure in Kg/mm2 (which is what the BHN is measured in) to lb/in2 (which is what we use for cartridge pressure). That is: conversion factor = 25.40 (mm/in) x 25.40 (mm/in) x 2.2046 (lb/kg) = 1,422.
There are several sources that say the best accuracy for a plain based lead bullet is when the pressure of the cartridge is 90% to 100% of the strength of the bullet so the final formula would be
BHN = ( Cartridge Pressure * 1.1) / 1,422 or Cartridge Pressure = BHN * 1422 * .9
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Thank you for taking the time to write this post. I've seen these formula's before and never really understood them. They've just always seemed like over kill to me.
A 35,000psi load:
9mm/357's/44mags/40s&w/10mm/38super/357sig
35,000/1422 ='s 24.6
24.6 x .9 ='s 22bhn
Lyman #2 alloy ='s 15bhn
Hardball alloy ='s 16bhn
linotype ='s 19bhn
monotype ='s 26bhn
From that formula you'd have to use a bullet cast out of 50/50 monotype (26bhn), and linotype (19bhn) for any of those pressures/calibers.
Something I learned awhile back (actually decades ago/getting old). A chronograph will tell you when you have the right bhn/alloy for the load your using. The velocity will fall off when you use too hard of an alloy for the load.
Simple enough to test:
Cast bullets out of 10bhn/12bhn/14/bhn/16bhn alloys and use the same lube and size them the same. Use the same load and run the different hardness of bullets over the chronograph. You will get to the point that the bullets too hard to seal the bbl and velocity will fall off.
The worst bbl leading I ever experienced was with a 22lr of all things. I was out plinking with an old/well used 4-screw k22. I went from hitting 6" targets to struggling to hit the broad side of a bard real quick. Took a look down the bore and shook my head. When I got home I used a tight patch on a plug and ran it down the bbl. This is what came out.
Anyway, just something to think about.