Shooting Cast Bullets at High Velocities
By Jim Green
For a very long time, I read a lot of modern load books that showed cast bullets being shot at low velocities. A lot of people told me that cast bullets were not capable of being fired more than 1600-1800 feet per second. If you look at the load data given for cast bullets in most reloading manuals, you'll see the data published for cast loads being small amounts of extremely fast burning powders. Sure this might be economical (small powder charges) but these loads operate at high pressures.
All cast bullets regardless of the alloy content, have a pressure limit. When you exceed this pressure limit, the bullet deforms to the point that leading occurs and accuracy goes out the window. The real trick to shooting cast bullets with success is to find a load that never exceeds the pressure limit of your bullet.
It's fairly easy to determine the pressure limit. I use a SAECO hardness tester to determine the hardness of the bullets using a number on the scale called the Brinell Hardness Number (BHN).
For my .30-06 rifle (model 1895 Winchester lever gun) I cast a bullet with a linotype alloy. I heat treat them by dropping the bullets straight from the mold into a container of cold water, to get a BHN of about 31. I swage a copper cup to the bullet's base to protect it from flame cutting by the burning powder gasses. I multiply this BHN of 31 X 1422 to obtain a maximum strength of bullet alloy in pounds per square inch, PSI. This gives me 44,082 PSI. So I look in my 47th edition of my Lyman Reloading manual I see a problem... The load data is given in the form of Copper Units of Pressure, CUP.
So there's one more handy math formula to use. Converting CUP to PSI can be done in the range of 30,000 to 50,000 CUP with about a 3,000 PSI error, so you should be cautious doing so. The formula is :
PSI = - 17902 + 1.51586 X CUP
Since I cast a 200 grain projectile, that's the first place I look for load data. I have a lot of IMR 4064, and IMR 4831 handy so I start there first. 47,400 CUP is maximum pressure for IMR 4831... Multiply 47,400 X 1.516 - 17,902 to get a total of 53,596 PSI too much for the cast bullet. A starting charge shows a velocity of 2323 fps, with a CUP of 35,800. SO.... 35,800 X 1.516 - 17,902 gives a measurement of 36,370 PSI.
This is well below the 44,082 PSI threshold for the cast projectile. So this is my starting point, 50 grains of IMR 4831 with a 200 grain heat treated cast bullet. You should keep in mind that the load data published by Lyman is for a 200 grain copper jacketed bullet at 2323 fps - NOT a cast lead projectile. I load a few test bullets at the starting charge, and gradually increase the powder by 1/2 grain increments, all the while I watch for signs of dangerous pressures on the cartridge itself. I look for flattened primers and check the fired cases with an RCBS Case Master gauge to ensure there's no incipient case head separation. I also check to see what powder charge gives me the most accuracy with this cast projectile.
Obviously the slower the burn rate of the powder, the lower the pressure. Maybe in the future I'll use a slower burning powder than what I have currently on hand. As mentioned earlier, each bullet has a pressure limit. By staying right around 90 % of that limit, this gives the optimum results. Richard Lee in his 2nd edition reloading manual, states that at 90 % pressure a bullet will slightly compress during ignition then return quickly to it's original shape. This 'plastic' compression allows the cast bullet to obturate during peak pressures and seal the bore. This prevents gasses from blowing by the projectile and eliminates gas cutting and leading of the rifling.
This may sound like a lot of work, but I get good results with high velocity and cast bullets. I get more than reasonable accuracy and at two cents or less per projectile, I get some very economic shooting time at the range. The great thing is the fact that there's no leading of the bore, the gun is easy to clean, and shooting cast lead bullets - there's virtually no wear on the bore of the gun.