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Originally Posted by ammo.crafter
Concerned about leading or not grabbing rifling, match BHN to velocity.
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So a shooter has two guns in the same chambering shooting the same lead bullet load. One has a barrel 40% longer than the other. Which one's velocity does he match the BHN to?
As described in detail by Richard Lee in Chapter 10 (p.129) of the second edition of his book Modern Reloading, it is peak pressure that determines when bullet distortion starts to occur for a given BHN. Lee shows that because the Brinnel testing method can be reduced to PSI of indentation force, if your peak chamber pressure never exceeds 1422 psi times the BHN number, regardless of what final velocity you get, the bullet will not undergo pressure distortion. That's very hard compared to what many use in rifles, and there are examples of a bit of deformation actually being beneficial, as when recovering bore obturation after passing through a bore constriction. Elmer Keith, for example, developed the 44 Magnum with BHN 11 bullets, but his pressures were roughly 2½ times higher than Lee's distortion point. So it's not an absolute limit, but one that tells you where bullet distortion will start, and you then have to determine how much of a factor that is for you. For the BHN 12, 16, 18, and 21 bullets commonly advertised, these pressures would be 17,064 psi, 22,752 psi, 25,596 psi, and 29,862 psi, respectively.
The 94:3:3 alloy, if not heat treated, is estimated by the
Cast Boolits site calculator to be BHN 12.2. That expanding bullet can't be too hard in order not to be too brittle, but just under 17,350 psi is where it will start to distort from pressure and is where you want to watch for an accuracy decline or a metal fouling increase. It's a pretty low number for a rifle, and even a starting load of Trail Boss makes more pressure, so some distortion is likely unavoidable. About the best you can do is watch for accuracy deterioration as you work the load up.
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Originally Posted by jamaica
Please do not do this. The 30-30 needs .308 bullets.
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As already explained, this is not true of cast bullets. Even for copper jacketed bullets, the SAAMI standard calls for bullet diameters of .306" to .309". Softer lead can easily be funneled into a smaller bore. Indeed, many 45 Colt guns from the 0.454 to 0.451 transition period have 0.451" barrels with 0.456" chamber throats, and bullets at the full 0.456" often shoot most accurately in these guns. Empirical testing has shown many lever guns like lead bullets at 0.002" over groove diameter for best accuracy.