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Colonel Harrison on bullet hardness

Posted: Sat Aug 08, 2015 3:25 pm
by mtngun
I recently re-read the NRA Cast Bullets book and picked up a couple of details in the fine print that I hadn't noticed before.

On page 144, there is a footnote that says
Continued experience indicates that linotype alloy, BHN 20-22, generally improves the reliability of target grouping in even these very light loads.

That agrees with my experience that hard bullets are good and that obturation is not necessary as long as the bullet diameter is correct to start with. :)

On page 14, there is an article on Is Jumping The Rifle Likely?. His analysis is rather crude, to do it correctly would probably require computer modeling, but that is not what caught my eye. Harrison's model estimates the stress on the bullet due to the rifling is 18,200 psi, and he concludes:
this value is within the compressive strength of even moderately alloyed lead.
That is a false statement. In reality, even hard lead alloys like linotype are only about 10,000 psi strength.

Page 16 seems to explain where the good Colonel got the false impression that lead is stronger than 18,000 psi.
Brinell hardness numbers express compressive strength in kilograms per square millimeter (each number then corresponds to about 1420 psi).
Once again, this is false, or at least it is half false. It is true that BHN is a certain measure of kilograms over square millimeters, but it should not be confused with strength. Even though they have the same units, BHN and strength are different tests that give different results and mean different things. They do correlate, but the correlation is strength = 500 X BHN, not 1422 X BHN as the Colonel suggests.

So the Colonel may be the original source of the 1422X myth? Other people attribute the 1422X rule to Richard Lee or to Veral Smith, but Harrison's article was published around 1963, long before Lee and Smith published anything. In any event I agree with Harrison that hard bullets are good and never mind the 1422X nonsense. :lol: