Brinnell Harness Number, or BHN as it is abbreviated, is a relative hardness scale. The numbers are derived from the load bearing ability of of a sample in terms of Kilograms (Kg) supported by one square millimeter (mm˛). This scale is mathematically progressive, in that tripling the BHN number, triples the sample hardness. Conversely, if you cut the BHN number in half, the hardness of the sample is half that of the original.
OK, what does that mean to the shooter and handloader? It is the most common measurement of hardness that is applied to projectiles. Here are some common examples:
Bullet Alloy 
BHN 
Pure Lead 
5 
1:20 TinLead 
10 
Wheel Weight 
11 
1:10 TinLead 
11˝ 
Lyman #2 Bullet Alloy 
15 
Linotype 
22 
Pure Copper 
40 
Now, since the relative BHN of an alloy can be definitively determined, it also stands to reason that the amount of force necessary to deform, or obturate the sample in question also may be calculated very precisely as well. For the purposes of handloading and shooting, this amount of force is most useful when calculated as pounds per square inch (psi.) necessary to deform, or obturate the base of our bullet.
To calculate the necessary psi. to obturate the base of a bullet, simply multiply the BHN of the bullet, by 1,422. This simple calculation results in the necessary pressure in psi. to obturate a given bullet.
Let's apply this to a bullet of BHN 18. (18 x 1,422) = 25,596 psi required to obturate the bullet!
Now, applying this method to the bullet alloys in the chart, we can derive the chamber pressures necessary to obturate the bases of bullets cast out of each one.
Bullet Alloy 
BHN 
PSI

Pure Lead 
5 
7,110 
1:20 TinLead 
10 
14,220 
Wheel Weight 
11 
15,642 
1:10 TinLead 
11˝ 
16,353 
Lyman #2 Bullet Alloy 
15 
21,330 
Linotype 
22 
31,284 
Pure Copper 
40 
56,880 
We have provided this information for those who desire bullets that obturate at their base for purposes of enhanced accuracy, when not fitting bullet diameters to those of actual cylinder throat dimensions. Many bullet manufacturers do not provide the custom sizing options that Beartooth Bullets makes available to every customer. As a result, you might have to settle for a "generic bullet size" that does not match your revolver cylinder throat dimensions. In that case, these calculations are essential to determining proper load pressures for your particular bullet hardness in order to tweak the load for best accuracy.