Low Pressure & High Velocity with Cast Bullets
Andy Moe
November 15, 2003
If the prairiedog had known what was coming next he probably would have stayed down his hole.
It was a clear day, no wind, and my custom Remington 700 .222 was in fine form. From 200 yards out I held the crosshair just under his chin and touched off the round, catching the dog in the chest with a 58 grain cast bullet launched at 2690 ft/sec from a charge of 23 grains of BLC-2 powder. The results were what you might expect from such a meeting. That high velocity cast bullet terminated the dog as well as if it wore a copper jacket.
Twenty seven hundred feet per second? If you’re not used to seeing that velocity figure tucked in behind the words “cast bullet” you’re not alone. For many years now the too-often repeated "Rule" for cast bullets has been that you can’t drive them much past 2000 ft/sec or you’ll get leading. While this Rule has dominated press and lore for almost half a century, growing numbers of knowledgeable shooters have been ignoring that rule and still meeting with great success.
There’s a new Rule in town now. It ignores velocity. Instead, the new Rule concentrates on pressure and is based on a simple truth: pressure causes cast bullet failure, not velocity. The overriding factor in cast bullet shooting will no longer be the velocity the shooter gets, but how one gets there.
Looking back on the majority of “cast bullet” data published over the last 50 years, you can see why the 2000 ft/sec level was set as the top end of the velocity range. Most of this loading data was derived from small charges of extremely fast burning powders. Green-Dot, Red-Dot, and Unique were at the top of the list. It doesn’t take very much of any of those powders to drive a 30 caliber cast bullet to 2000 ft /sec. Unfortunately, they do so at a pressure of 40,000 psi.
Economical? Sure. But these pressures are just too high for most bullet casting alloys. All cast bullets, no matter what the alloy, have a pressure limit. When this pressure is exceeded, the bullet to compresses to the point where it becomes “plastic”. I refer to this limit as the “yield strength” of the alloy. It’s at that point that leading occurs and accuracy becomes non-existent.
The trick to successful cast bullet shooting then is to find a load for your bullet that never exceeds the yield strength for your particular alloy. This yield strength is easy to determine once you know approximately how hard a given bullet metal is; and determining the hardness is relatively simple as well. You can either estimate the hardness by casting from a known quantity such a pure linotype, wheel weight, or pure lead; or you can obtain a hardness tester with which you can test your alloys. There are several hardness testers on the market for testing lead alloy: LBT Industries, SAECO, and Lee all produce testers. The prices vary from $40 to $100 but the results should be reliable with whichever brand you choose. These devices will determine the hardness of your bullet on the Brinell Hardness Number scale, or “BHN” for short. The BHN measurement is necessary to calculate the amount of pressure a bullet will withstand.
I confess, don’t have a tester. I’ve gotten around this lack by loading heat-treated bullets cast of a 9:1 mix of wheel weight and linotype. These bullets always have a BHN of around 31, and while they will soften slightly over time, they remain quite hard even then. To give you an idea of relative hardness; heat-treated bullets have a BHN of 31, linotype has a BHN 22, wheel weights are around 10, and pure lead has a BHN of 6. These figures for the non heat-treated bullets are approximate but close enough for general use. The hardness of your alloy will determine what pressures you have to work with to obtain your velocity goals.
If you have a BHN value for your favorite cast bullet you can now put that number to work for you with a bit of simple mathematics. If you multiply that BHN number by 1422 you will have the maximum yield strength of your alloy in pounds per square inch, or “psi.” For my heat-treated bullets with a BHN of 31 the results are always the same: 31 X 1422 = 44,082 psi. By the same calculations we get 31,284 psi for linotype, 14,220 psi for wheel weights, and 8,534 psi for pure lead. Armed with the yield strength of your alloy you can now go to work selecting loads.
The first thing you’ll notice is that most loading data is given in “C.U.P.”, or copper units pressure, not in PSI. According to the NRA publication “Measurement of Chamber Pressure”, by Wm. Davis, the reason that chamber pressures are measured in CUP stems from a systemic error in crusher-type pressure guns. Lost energy between the chamber and the crusher piston allows for a pressure reading between five and twenty percent less than actual peak chamber pressure as measured in pounds per square inch. (This is perhaps an over simplification as the cartridge shape and bore diameter of the cartridge seems to have an effect on the PSI/CUP variance. In some cases the CUP is higher than the actual PSI transducer reading. There is little to explain these anomalies but the 5 to 20 % figure seems to be the case for many cartridges and again, offers a margin of error to prevent bullet failure.)
This has been borne out in comparative tests using electronic transducer pressure readings. It was this discrepancy between actual peak pressure and the pressure calculated from the crusher that prompted the term “CUP” for pressure readings instead of the more applicable “PSI”. In deference to the lesser pressures generally registered by CUP readings, I like to reduce my yield strength by 5 percent across the board. This leaves my heat treated cast bullets with a yield strength of roughly 42,000 psi or CUP. With that allowance made we can usually equate the terms and still allow for a good margin of safety from bullet failure and subsequent leading. You will find that best accuracy comes from a load 10% below yield strength anyhow: the “yield strength” is after all, the point at which the alloy fails. Common sense tells us that successful loads will have pressure levels below that figure. Knowing what you now know about the yield strength of your bullet’s alloy you can select appropriate starting loads without fear, provided that you have a reloading manual that lists chamber pressures for your cartridge.
To illustrate the process, I’d like to use that .222 Remington as an example. I developed the load for this rifle 10 years ago using the then-new Lyman #47 reloading manual. In choosing my data source, I skipped over the “cast bullet” data for this cartridge and went straight to the jacketed bullet section. Under the 60 grain load (remember, I was shooting a 58 grain bullet so I wanted to err to the heavier side when picking my data) I went down the right hand column under “Pressure” and found several pressures close to my yield strength of 42,000 psi / cup. It was as simple matter of choosing one of the powders associated with these acceptable pressure levels.
At the time, my powder choice was dictated by what was on hand and in this case, BLC-2 was the winner. I had a fresh pound of it waiting unopened. The data showed a maximum charge of 24.1 grains yielded pressures of 43,100 C.U.P. and a velocity of roughly 2800 ft per second. I began by loading 20 grains, which, according to the manual, would yield a pressure of 28,200 C.U.P. This was well below my alloy’s yield strength. Chrongraphing revealed a velocity of 2310 ft./sec and there were no pressure signs. Accuracy was about 1.5 inches at 100 yards. I proceded to work up my loads in half-grain increments. At 23.0 grains I was getting good, usable velocity and no leading. The accuracy was in the ¾ - 1.2 MOA area for 5 shots. I stopped there.
It should become obvious by now that the slower the powder, the lower the pressures. Had I a supply of H-4895 on my shelf, I’d have certainly gone with that powder first. Looking at the loading data for that powder, a maximum charge of 23 grains delivers 2750 ft/sec with a pressure well under that 42K mark I set for my bullets. It would have been a no-brainer for cast bullets.
Later tests with this powder showed that it was ideally suited for this cartridge and bullet. Twenty-one point five grains of this powder delivered MOA and 2642 ft/sec. No pressure signs. No leading. Another good choice for this cartridge would have been Winchester 748. In this Lyman manual it is one of the few powders listed under cast bullet loads to develop good velocity with minimal pressure. This .222 is just one example of the many different cartridges in which I have utilized high speed/low pressure cast bullet loads. A survey of any good loading manual will show the reader that with few exceptions, there can be loads of this nature found for almost every cartridge for which cast bullet molds are produced. I have loaded 22 Hornet through 375 H&H and loading to pressure has never let me down.
As mentioned earlier, the “yield strength” of the bullet is the maximum amount of pressure a bullet can take. Serious cast bullet shooters have found that staying around 90% of that figure usually gives optimum results. While it was generally thought that a maximum pressure load was just putting the bullet “on the ragged edge of destruction”, Richard Lee, in his Second Edition loading manual states that at 90% of yield strength the bullet will compress during ignition and then return to its original shape. Add more pressure and the bullets’ return from that compressed state diminishes to varying degrees with the amount of pressure applied. It’s a reasonable assumption and one that has been borne out by many years of practical application. Another thought with regards to that 90% target pressure is that during the compression phase, the bullet seals the bore at peak pressure, returning to it’s normal state as pressures drop. This obturation will certainly aid in preventing gas cutting and leading.
You have probably noticed by now that the loads you get from from this method are neither the fastest, nor the most efficient. That’s the small price we pay for shooting lead bullets. There will be those who feel having a fine modern centerfire rifle and not shooting at the highest velocities obtainable is a waste of their hard earned money. I won’t argue with them. Cast bullet shooters are generally a different lot. These are folks who are either inquisitive enough to want to do it, have decided that bullet casting will save them a bit of cash, or like myself, have been down that high velocity road already and don’t feel the need to make the journey again. All seem to feel that the casting and load development are worth the satisfaction they get when everything clicks into place. To drop a head of game with a homemade bullet is quite a thrill. I have killed vermin, varmints, deer, and buffalo with them. No game I hit has ever seemed to notice the lack of a copper jacket, and none ever complained that I was too frugal with the powder charge. At about 2-cents per bullet you can make the case for economy as well.
Cast bullet shooting has come into a new era. With the velocity limits of cast bullet loads redefined by chamber pressure, the modern shooter will find the performance gap between jacketed bullets and cast bullets narrowing, and the horizons for practical application of the lead alloy bullet widening.
~ Andy Moe
JHO field writer & pro staff
