jwp wrote:Brinell Hardness is measured in kilograms per square millimeter of the spherical surface area of the indentation made by the ball used for testing. One kilogram is a about 2.2046 pounds, and one square inch is about 645.16 square millimeters. Thus, the conversion factor between BHN and PSI is approximately 1422.32, and a BHN of 30 (kilograms per square millimeter) equates to about 42670 PSI.
Most metals have the same strength in tension as in compression. On the other hand, concrete is much stronger in compression than in tension. But most of us don't shoot concrete bullets.He is specifically concerned with the relationship between the ultimate compressive strength of the bullet ,
The Fouling Shot published a study of loads used in CBA competition over the coarse of a year (CBA rules require competitors must publish their loads including the bullet alloy). The author used Quickload to estimate the chamber pressure for each of those loads, and tried to find a correlation between load PSI and BHN. There wasn't one.What Mr. Lee and his son seem to have found through testing was that the best accuracy occurred when the chamber pressure (in PSI) was around 90% of the bullet's ultimate compressive strength (as obtained by multiplying its BHN by 1422).
Gas checks will expand to fit the bore at less than the tensile strength of gilding metal. In fact, some of my customers use gas checks on muzzleloader bullets, sizing them for a slip fit in the bore, and relying on obturation to bump up the bullet, including the gas check, to fit the groove.53K to 61K PSI you reported far exceeds not only the tensile strength of lead, but also that of copper gas checks.
No secret that it is easier to get good accuracy with cast bullets at lower velocities and pressures. This is not news.More interesting, is that when you lowered the pressures, accuracy improved.
[Emphasis added, and for our purposes I think we can happily ignore megapascals as a unit of measure.]www.calce.umd.edu wrote:The yield strength in tension is about 1/3 of the hardness . To find the ball park figure for the yield strength convert the hardness number to MPa (or psi ) and divide by 3.
mtngun wrote:If we are foolish enough to shoot undersize bullets ... [w]e want our undersize bullet to obturate almost as soon as it begins to move, when chamber pressures are well below peak. That makes it next to impossible to come up with any simple formula for predicting the peak chamber pressure required to provide satisfactory obturation. Perhaps the 1422 x BHN rule is as good a rule of thumb as any for that purpose ?
mtngun wrote:My preferred solution is to size the bullet to fit the gun and then not have to rely on obturation, and not have to worry about BHN.
But there is no significant difference between compressive strength and tensile strength for most non-brittle metals. Engineers use them interchangeably.jwp wrote:Neither I nor Richard Lee claimed that HB*1422 = tensile strength.......Richard Lee thinks HB*1422 gives the ultimate compressive strength in PSI.
The formula used to calculate the pressure required for solid base bullets is: Bullet's BHN x 1422 = pressure in pounds per square inch
The constant 1422 is a mathematically derived number.
atomictaco wrote:OK, I'm just starting in the bullet casting hobby and trying to determine the proper BHN for 9 x 19. Based on this thread I've come up with a BHN of 84.3? Using PSI=375 x BHN +500
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