Jim,
Your question is a common one, and no simple answer will suffice. I took the liberty of copying some of the information on this subject from our FAQ pages for your reading here; this should help quite a bit in answering your question!
God Bless,
Marshall
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Quote </td></tr><tr><td id="QUOTE"><!--QuoteEBegin-->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 (mm²). 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 Tin-Lead 10
Wheel Weight 11
1:10 Tin-Lead 11½
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 Tin-Lead 10 14,220
Wheel Weight 11 15,642
1:10 Tin-Lead 11½ 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.
<span style='color:red'>What About The Idea Of Bullet Base Obturation Being Necessary To Revolver Accuracy?</span>On 2001-01-29
This idea has merit when the bullets do not properly fit the cylinder throats of a revolver. The concept relies on this premise: The bullet is undersize for the cylinder throat, and therefore when the bullet hits the forcing cone, and is still in the mouth of the chamber of the revolver, expanding powder gasses leak around the base of the bullet when it hits the resistance encountered at the forcing cone. This leaking gas will cause "gas cutting" at the heel of the bullet, thus severely deteriorating the potential accuracy of that bullet. The theory, or practice here, is to use a bullet hardness that is matched to the pressure generated by the given load in order balance the pressure necessary to obturate (read deform) the base of the bullet to totally seal the chamber mouth before the bullet hits the forcing cone, thus preventing the "gas cutting" and enhancing potential accuracy of the load.
This concept does work to a degree, and can greatly enhance the accuracy of some load/bullet/revolver combinations. However, it is flawed in its conception of being the perfect scenario. Please consider that if the bullet is undersize for the cylinder throat, that it will most likely be laying at the bottom of the chamber (Yes, at the bottom of the chamber! Remember that the chamber has to be bigger than the cartridge for it to easily chamber, and that the cartridge will lay at the "bottom" of the chamber, even if it is just a few thousandths of an inch!) upon ignition of the cartridge. Consequently, when that bullet, that is undersize for the throat of the chamber, obturates under pressure of the expanding powder gasses, it is already out of alignment with the forcing cone and the central axis of the bore by a few thousandths of an inch. Now, considering this aspect, the bullet will already be against the bottom part of the throat of the chamber once the bullet's base obturates (deforms), the odds of that bullet uniformly expanding at its base are really unthinkable. It will expand into the unfilled space of the throat, and thus the bullet will remain slightly off center in relationship to the central axis of the bore, and will remain that way throughout its passage through the barrel! Yes, this bullet will most certainly shoot better than if it did not obturate, in that the powder gasses were sealed in the chamber mouth and the base of the bullet did not become eroded from escaping gasses. This concept does have its limited merits.
Now, consider a more precise, and predictable alternative. If the exact dimension of those chamber throats in the cylinder are known, from slugging and measuring, then a bullet of the exact same dimensions can be procured to fire in that particular firearm. This being the case, the bullet will be a firm, snug slip-fit through the throat of each chamber in the cylinder. With a bullet that already tightly fits the chamber throats, it is not necessary for the bullet to obturate in order to seal the mouth of the chamber when that bullet hits the forcing cone and pressures climb. It fit the throat BEFORE firing! Carrying this one step further, the loaded cartridge, with a bullet that is a snug slip-fit in the chamber throats, will automatically center itself in the chamber, due to the tight dimensional relationship between the bullet and the chamber throat. No longer does that loaded round lie in the "bottom" of the chamber, but rather is centered by the bullet in the chamber throat, establishing near perfect alignment with the central axis of the bore, (assuming good cylinder timing and bore alignment), before the projectile is even launched. The result is much more predictable ballistic performance and more forgiving load development, as well as reduced leading in the forcing cone and rearward portion of the barrel.
This is the main reason that Beartooth Bullets stresses custom sizing for our customers, and encourages each of our clients to determine the exact dimensions of their firearm chambers to ensure proper bullet size application. When this dimensional relationship described above is properly balanced, bullet obturation no longer plays a role in bullet accuracy or load development. The bullet will already fit the gun, and not need "pressure fitting" by obturation (read deformation).[/quote]
