11 Sep

Cast bullets, obturation and our parting thoughts.

 

Cast bullets, obturation and our parting thoughts.

In our opinion all the hype surrounding bullet obturation is nothing more than hype.  There are so many versions of the “formulas and facts” that differ so greatly they all can’t be true.  How do you pick out the truth?  You test the theory yourself like we did and see what the end results tell you.  Here are a few of our observations surrounding the hype and reasons to question it.

·         If bullets are properly sized, at least .001 over bore diameter, they are being squeezed into the barrel.  No further obturation is necessary as the bullet has obturated or “sealed the bore” because it is of a larger diameter than the bore to start with.    

·         Nowhere does the formula for obturation state when the calculated pressure must occur.

·         Nowhere does it state the amount of obturation to expect.  Are they talking .001 or .0001? 

·         Are they expecting obturation just at the base or is it carried thru the entire bullet?  If the obturation is just at the base how can it work on a bevel base bullet?

·         How would obturation swell the bullet evenly around the entire circumference?       

·         Maximum pressure isn’t instantaneous so it isn’t like striking a cast bullet sitting on an anvil with a hammer as some websites state.  The bullet on the anvil can’t move but the bullet going down the barrel moves with relative ease.

·         Reloading manuals don’t state any obturation formula in them.  If obturation were required you can bet all the loading manuals would reference the formula and incorporate it into load requirements. 

·         Many cast bullets made from 2/6/92 and harder alloys would never obtruate because to reach the necessary pressure, calculated by the obturation formula, you would have to exceed maximum recommended loads.

·         One bullet manufactures website states the following formula:  Optimum BHN = C.U.P./(1422x.90).  C.U.P. and PSI are not the same and there are no reputable charts or formulas that we can find to convert from one to the other.  Again if this formula existed it would be in the manuals and it isn’t. 

·         Another website maintains obturation is BHN x 480.  This is quite different from the formula commonly stated on the internet.  Implies obturation happens at much lower pressures than the other formulas do.

·         Wikipedia defines the formula as BHN x 1422 = pressure in pound per square inch.  It also states the formula should only be used with cast lead plain base bullets.    

·         So which formula is correct?  Are any of them correct?  Best case scenario would be only two of them are wrong!

·         Modern Reloading Second Edition by Richard Lee has a section on Matching bullet metal to chamber pressure.  Article doesn’t use the term obturation or the obturation formula but does use Brinell hardness.

In conclusion there isn’t sufficient evidence or strength of argument to sway us to believe that bullets, undersized to the bore, will obturate and seal the bore. If obturation happened it would have worked on our undersized bullets and it didn’t.