Cast bullets and obturation, fact or fiction?
Much has been written on the necessity of a cast bullet to obturate to “seal the bore” and keep gasses behind the bullet. You can find formulas for the amount of pressure necessary obturate a cast bullet based on the Brinell hardness, BNH, of the bullet. Obturation seems to be an obsession with a lot of cast bullet shooters so we decided to investigate the matter and throw in our two cents worth.
The most common stated obturation formula is the cast bullets Brinell hardness number multiplied by 1422 equals the amount of pressure in PSI necessary to obturate the bullet. Bullet BHN x 1422 = necessary PSI for obturation. Our first thought is related to obtaining the correct Brinell hardness number of the alloy. Let’s examine the results of being off on the hardness.
12 * 1422 = 17,064 As you can see if your Brinell hardness is not accurate it will have a
13 * 1422 = 18,486 great effect on the theoretical pressure you need to obtain. Los Angeles
14 * 1422 = 19,908 Silhouette Club did an extensive test on hand held Brinell hardness
15 * 1422 = 21,330 testers. Check out the results for yourself!
16 * 1422 = 22,752
17 * 1422 = 24,174
Now browse thru your manuals and see who lists load pressures in PSI. The only manuals I have listing pressure in PSI is Lee. Normally I like to compare load data in two manuals as the data can vary. I have not found a conversion chart from C.U.P. to PSI in the loading manuals. I don’t believe there is such a conversion chart based on my research.
Another unexplained portion of the obturation theory is how you determine the exact time in the powder burning cycle your target pressure is produced and when it needs to be produced. The stated purpose of obturation is to seal the bore so we assume it has to happen before the driving bands are in the barrel. Is it inside the cartridge? Perhaps it is in the area between the chamber and rifling.
We recommend that you shoot cast bullets sized at least .001 over actual bore diameter. The cast bullet is then squeezed into the bore making obturation unnecessary.
As an experiment we performed a test to check for obturation. Since our standard alloy, 2/6/92 (Brinell hardness of 15 according to Lyman), would fail to obturate we added pure lead to the foundry alloy and estimated a BNH of 12 – 13 on the bullets cast from it. We slugged the bore on a 45 ACP pistol and determined the actual bore diameter to be .450. We then turned the driving bands on four 200 grain semi-wadcutters down to .449. Since it is .001 under actual bore diameter obturation would expand the cast bullet to .450 and seal the bore.
Using the Second Edition of Modern Reloading by Richard Leeas a reference manual we chose a maximum load, 5.5 grains, of Winchester 231 which lists a pressure of 19,600 PSI. Our estimated 13 BNH calls for 18,486 PSI to obturate the base of the 45 acp bullets.
What we discovered was not obturation but rather a .002 reduction in diameter of the driving band below the lube groove and a .001 reduction in diameter of the driving band above the lube groove. The barrel with only four shots fired was full of powder residue. Gas cutting marks were visible on the entire circumference of the bullet except for in the lands.
In our next blog we will discuss more on the theory of obturation.
Warning: Any content and data contained in this article is for informational purposes only. Always consult recognized reloading manuals for load data. Any load data or testing by Central Plains Enterprises, LLC is not an approval, endorsement or suggestion for readers to attempt to replicate. We disclaim any and all responsibility, implied or expressed, for any person using data mentioned or replicating testing done by us.