Cast bullets for hunting--and why not?
The forebears of modern cartridge firearms of course were the muzzle-loaders--and they left their mark in Africa as they did elsewhere around the world. But the modern sabot for front-feeders only came on the scene circa 1985, courtesy of Del Ramsey--and this development took away most of the limitations associated with cast bullets for hunting. And then, of course, came a more widespread use of jacketed bullets, and when we jump to cartridge firearms few hunters would consider using anything other than jacketed projectiles, especially on dangerous game.
Now, on the plus side, pure lead maintains the molecular cohesion--that is, the resistance to breaking up--that has always made it ideal for casting into bullets, and it also expands reliably across a wide spectrum of velocities resulting in an expanded mushroom inside the body cavity of a game animal. And it is inexpensive.
The converse is that no matter what bullet material or configuration you select, all bullets have limitations, and soft lead is no exception. Muzzle velocities above 2,150fps are close to the velocity limit above which the lead itself can deform in flight due to nose pressure, and of course the softer the lead the more bore-leading one can expect. Cast bullets can exhibit serious flaws, such as uneven cores and void, empty spaces, which can be overcome by swaging the bullets--but most folks that I know here in Africa continue to cast their bullets the old-fashioned way.
So, though the tendency has been to use jacketed bullets in high-powered rifles, a bullet cast or swaged from slightly harder lead and with a gas check--that cartridge equivalent of a sabot--affixed is certainly a viable alternative. And it can be a lot more fun, too.
There are a number of different methods of determining the relative hardness of a metal, but the one used to rate the hardness of lead in reloading circles is the Brinell (BHN) scale, introduced by the Swedish engineer Johan Brinell in 1900. It works by measuring the indentation on a sample caused by a known force applied through an indenter. Though it is less well-suited to harder metals, it works ideally with lead, where a BHN of 5.0 denotes pure lead, and various lead alloys can go up to 35.0.
Hardness testers are not overly complicated, and are available to any reloader or bullet caster--perhaps one of the most common is the one that we use at my shooting club, the LBT Hardness Tester. It is solidly constructed, and measures BHN from 6.0-40.0. It takes you about ten seconds to test a cast bullet, and the results are guaranteed accurate to 1 BHN. Lee also offers a test kit that is designed to be used with a reloading press, and there are others.
But even harder lead alloys, while still easy to work with in terms of bullet casting, will still leave lead deposits in the bore at higher velocities. I have a seemingly mirror-like finish in the bore of my Colt .45 pistol, but firing cast bullets at even moderate velocities of between 830-850fps will leave annoying lead deposits in the bore. Which is why I shoot Frontier FMJs in competition. But I digress. The solution is the gas check, for which, of course, you need a gas-checked bullet mould.
A gas check is a thin cup, usually crafted from copper, zinc or aluminium alloy which is affixed to the base of a lead cast bullet during the reloading process. Generally, gas checks are used with higher-pressure and velocity cartridges to prevent the buildup of lead in the barrel which enhances accuracy. In order for a bullet to be fitted with a gas check it requires a slightly rebated area at the base to allow the gas check to be pressed in place whilst the bullet retains the same diameter, and so the modification has to be incorporated into the mould.
The gas check is effectively swaged onto the bullet, covering most or all of the base, during the lubrisizing process. Once fitted, the gas-checked bullet is loaded conventionally.
When a high-powered, or magnum round is fired, the high pressures allow propellant gas to escape past the bullet, causing gas cutting of the base which increases lead deposits in the barrel and may unbalance the bullet in flight. The function of the gas check is to afford a thin layer of harder but still-malleable metal on the base of the bullet that creates a seal to prevent the propellant gasses from leaking past.
The majority of reloaders, of course, will use jacketed bullets, though gas checked bullets are often less expensive, especially for a reloader who can recycle lead to re-cast bullets and then must only factor in the cost of the gas check. Custom lead bullets, such as those for obsolete calibres or wildcat cartridges are easily made with inexpensive casting or swaging equipment. Conversely, the manufacture of jacketed bullets requires far more expensive equipment to draw the jackets and swage in the core. This level of technology is generally limited to commercial ammunition producers.
So, for an effective hunting load, we're looking for a bullet with a BHN hardness of at least 15 and preferably greater, configured for a gas check. Oh yes, and a reloading manual that gives chamber pressures for the cartridge you're loading for.
Remember we said that the main goblin is pressure, not velocity? Well, in the past, reloaders tended to use small amounts of very fast-burning powders to garner higher velocities--and of course while saving money and seeming to get the performance they were after (in terms of velocity) they were getting very high pressures into the bargain. You no longer need a copper crusher to measure chamber pressure--this can easily be done on the range with Recreational Software's Pressure Trace apparatus. Because the pressures exceeded the limits of a cast bullet, leading would occur and accuracy would diminish. In order to keep your loads within tolerances of what the alloy will withstand, you need to know the hardness of the metal.
Once you know what your BHN value is for your alloy mix, you can multiply that figure by a factor of 1,422, and this will give you what cast bullet guru Andy Moe calls the bullet's "yield strength" in pounds per square inch. So there is no confusion between the implications of PSI relative to CUP, actual pressures in PSI are generally higher than what is indicated by traditional CUP, because there is energy lost between the chamber and the piston in the copper crusher used to obtain the values. Moe recommends reducing your yield strength figures by 5% to allow for what he considers an acceptable margin of safety from pressure-caused bullet failure.
Not all reloading manuals list chamber pressures for various cartridges, but once you've calculated your yield strength you have to know whether your alloy is within the parameters for a particular calibre. Lyman manuals do incorporate this information. As an example, pure lead will have a yield strength of 7,110 if you use the BHN value of 6, and I have a Savage M99 in .300 Savage that I want to shoot cast bullets in. The .300 Savage is a real piece of history when the US military decided to replace the old .30-06 round, the .300 was the prototype they chose to experiment with and it became the forerunner of the .308 Winchester. Being loaded to slightly lower pressures than the .308, the Savage develops around 46,000 CUP, or 47,000 PSI. I'm going to have to go for a pretty hard lead alloy to fit these parameter --say BHN 33 or above.
Or, of course, play around a little with bullet weights and charges and powders with different burning rates to try and adjust pressures. Some of the many things that have a bearing on chamber pressures and velocities are the burning rate and characteristics of the particular powder; the charge weight and to what degree this fills the case; diameter, weight and bearing surface of the bullet; barrel length; ambient temperature; free bore in the barrel, and neck tension holding the bullet in place. With all other factors within parameters, a 20% decrease in powder charge can result in a 60% decrease in chamber pressure. The slower the powder, the lower the pressures, and it is quite possible to generate higher velocities with lower pressures with cast bullet loads.
Will lead bullets ever be the fastest or the bestest? No. Is it worth persevering with them even in modern center-fire rifles? Yes. Edge-of-the-envelope loads are very rarely the most accurate, and to my mind there is far less satisfaction reloading by numbers than there is playing around with load development and making a cast bullet--your cast bullet, that you made all by yourself--do what you want it to. If that includes bringing home a load of venison or taking a good trophy, so much the better. Cast bullet shooters stand a little apart, but they generally have a deep and intuitive understanding of internal ballistics and of course by definition they fit into the category of gun-nut and maybe enjoy their shooting just a little more for it.