Materials for 50 Caliber Bullets
By: Daniel Lilja
This article was prompted by a desire to look into the differences in materials used in making solid, monolithic-construction, fifty caliber bullets. Most of the bullets made commercially and by experimenters have been made from one of several materials. The list includes brass, bronze, copper, and steel. Generally these bullets are made on CNC-type lathes or screw machines, and in the case of the steel bullets, Zero Index follows the turning operation with a center less form-grinding step. Another exception is the bullets made by Accurate Projectile Technology. These are made from cold-forming copper bar stock. I’m not sure how the Thunderbird bullets are made, but I think they’re cast from melted down cartridge brass and then
I’ll attempt to share what I’ve learned from shooting some of these bullets and from research I’ve done on the materials. I wanted to know what weight differences could be expected for bullets made from these various materials. And how did this weight difference affect internal and external ballistics? What, if any, effects on accuracy did each material offer?
Effects of Density on External Ballistics
These four different materials each have their own density. Their effect on the ballistics of bullets made from them is twofold. The gyroscopic stability of a bullet increases along with density, meaning that it takes less barrel twist, or bullet spin, to stabilize a bullet. From an accuracy point of view, that’s a desirable characteristic. Slower twists are more accurate, especially with poorer quality bullets. The second effect of bullet density lies with ballistic coefficient. This can clearly be seen in the formula for calculating ballistic coefficient: BC=SD/i
where: BC is the ballistic coefficient, SD is the sectional density of the bullet, and i is the form factor of the bullet.
Since high ballistic coefficients are desirable, we can see that if we increase the sectional density of the bullet, BC will rise. Alternatively, the form factor could be reduced. Form factor is dependent on bullet shape and this subject was covered very well by Eric Williams in VERY HIGH POWER 1993, issue #3.
Our concern here is material density and its effect upon sectional density. In turn, sectional density is defined:
where: w is the bullet’s weight in pounds, and d is the bullet diameter.
If we want to convert the weight from pounds to grains, the formula looks like this:
The number 7000 is a constant, since there are 7000 grains in one pound.
It’s obvious that if one material is denser than another, a bullet made from the denser of the two will weigh more. This increase in weight will therefore increase the bullet’s ballistic coefficient. The term specific gravity is a measurement of a particular material’s density. A value is assigned to all materials indicating specific gravity. For example, lead has a specific gravity of 11.34. Water has a specific gravity of 1.0 at 62 degrees F., and is used as the yard stick in measuring specific gravity for all other materials. This means that for a given volume, lead will weigh 11.34 times as much as the same volume of water.
A jacketed lead-core, soft-point bullet has a specific gravity from 10.25-10.4, depending on the ratio of lead to copper. A jacketed hollow point will measure from about 10-10.11. Now let’s turn our attention to monolithic solids. The specific gravities of these materials follow: copper 8.89, bronze, 8.78, brass 8.2-8.6 (depending on the alloy), and steel 7.64-7.85 (depending on alloy again). So by a small margin, copper is denser than bronze, followed by brass and then steel. Until I looked up these values, I thought that steel was quite a bit lower than the other materials. Of academic interest only, silver has a specific gravity of 10.5 and gold 19.3.
I wondered then, what would various bullets weigh, all the same size and shape, but each made from a different material from the list above? And what would each bullet’s ballistic coefficient be? To answer the weight question, I used a computer program I wrote in GWBASIC that calculates a bullet’s weight from its physical dimensions and specific gravity. An article describing this process and a listing of the computer code was printed in VERY HIGH POWER in the 1992, #2 issue.
Remember, the shape of these bullets stayed the same, therefore the form factor did too. For bullet shape, I chose one that is similar to the 700 grain, steel bore-rider offered by Zero Index. Actually this bullet weighs about 703 grains on my scale. Not coincidentally, the shape of the Mance 765 grain brass bullet is almost the same. There is a little difference in the driving band, but not much. The physical dimensions I plugged into the weight program are: a bullet diameter of .502″ (driving band of .511″), 2.500″ overall length, 8 caliber tangent ogive, a meplat diameter of .005″, and a boattail .240″ long with an end diameter of .410″.
I found that a copper bullet would weigh 809 grains, a bronze bullet 799 grains, one from brass 765 grains (like the Mance) and the steel version 703 grains (like the Zero Index). If we had more dollars than sense, a gold bullet would weigh 1757 grains.
Answering the ballistic coefficient question is a little more involved. I used a program written by Bill Davis of Tioga Engineering and Bob McCoy of Aberdeen Proving Grounds. This program calculates both C1 and C7 form factors and ballistic coefficients. It does so from specific inputs about the geometry of the bullet and its velocity. Throwing velocity into the calculations complicates our question here, because as we know, lighter weight bullets can be driven to higher velocities. Ballistic coefficients normally increase with velocity.
Effects of Weight on Internal Ballistics
So, before we can answer the ballistic coefficient question, we must first make some accurate assumptions about what velocities we can push from our four different density bullets. In another article I wrote for VERY HIGH POWER about velocities and barrel lengths, I mentioned an internal ballistics program that I used for predicting velocities. This program has been an accurate predictor of velocities, so I relied on it to help me out again. In the earlier article I just changed barrel length, looking for velocity differences. This time I left barrel length the same, 36″, and changed bullet weights. (I won’t go into detail again about how the program works; those who are interested or skeptical can look up that article.)
The internal ballistics program calculated that if we started the 809 grain copper bullet at 2686 fps, then a 799 grain bronze bullet would leave the muzzle 14 fps faster, at 2700 fps. To my surprise, it said the 765 grain brass version would be pushed to an even 50 fps more, or 2750 fps, and the 703 grain steel bullet would be an even 100 fps quicker, at 2850 fps. It’s unusual for velocities to be spaced at nice 50 or 100 fps intervals, but that’s what the computer said. These velocities are reasonable from a 36″ long barrel with a 50 BMG chamber.
Calculating C1 Ballistic Coefficient
Now that I knew both bullet weight and comparable velocities, I could put those numbers into the ballistic coefficient program. Of passing interest, I also put those numbers through another Tioga Engineering program that accurately calculates gyroscopic stability. This program determines which twist rate will yield a stability factor of 1.5 by using physical dimensions and specific gravity data, along with velocity. Without going into a lot of discussion about stability factors, a value of 1.5 is looked on as a conservative number, giving both accuracy and adequate stability for any normal conditions. If the stability factor drops below 1.0, this indicates that the bullet is not stable in that particular twist rate. By bringing this number into the discussion, we can see how stability decreases along with specific gravity.
I found that the copper bullet had a C1 ballistic coefficient of .992, and a 15.1″ twist rate gave a stability factor of 1.5. The bronze bullet’s ballistic coefficient was close, at .981, and a twist rate of 15.0″. The yellow brass bullet needed a 14.7″ twist, and the C1 value was .944. The magnetic bullet liked a 14.2″ twist rate, and had a BC of .879. Zero Index states that the ballistic coefficient of this bullet is about .89, so our number looks pretty close. These ballistic coefficient values are said to be accurate to within about 5%. They should show closer accuracy between each other however, because the form factor is constant. Looking at it another way, if one is off by 5%, they’re probably all off the same amount, making our comparison valid.
Putting it all Together
The big question should be, how does all of this theory affect a bullet shot at a target 1000 yards away? Ballistic coefficient and velocity affects bullets in two important ways, as far as the paper puncher is concerned. Those two things are bullet drop and–even more important in my opinion–wind drift.
Again, we can accurately calculate drop and drift at 1000 yards if we know bullet ballistic coefficient and velocity. And again, I relied on another Tioga Engineering program (TRAG1Q) to do so. The computer indicated that the copper bullet would drift about 39.39″ at 1000 yards in a 10 mph direct cross wind, and that the total bullet drop would be 308.1″ The bronze bullet would blow 39.62″ and drop 305.7″. As we would suppose, the performance of these two bullets is almost identical. From a practical point of view, they are ballistic twins. The brass bullet shows a drift figure of 40.41″ and total drop of 297.4″. To me, the interesting one is the steel bullet, drifting 41.83″ and dropping the least, at 281.8″. All of these numbers are based on a standard sea-level atmosphere. Increasing elevation or temperature will decrease both values.
Musings on Materials
I’ve also made some observations about the materials we have mentioned, not related to ballistics. One concern with all of the solids is barrel wear. In my opinion, the jury is still out on this subject. I’ll leave the discussions about various barrel treatments and bullet coatings to others. But I feel that with solid bullets, chrome-moly barrels will last longer. With jacketed bullets, such as the Hornady, McMurdo, and FMJ military bullets a stainless steel barrel is just as good and often will clean up quicker.
In my experience, copper bullets tend to be metal foulers in the barrel. However, with appropriate cleaners and periodic use, the shooter can normally stay on top of fouling. Material cost is higher than brass, but not out of sight. As we saw, copper also has the highest specific gravity and resultant sectional density. Copper can be challenging to machine, but APT’s cold-forming operation eliminates this problem.
Bronze is the most expensive material of those we mentioned (not including gold, of course). It’s also the most slippery, having the best friction coefficient. I’ve fired some bore-rider bronze bullets in a stainless steel barrel with virtually no fouling. The specific gravity for bronze is close to copper.
Brass is the least expensive of the non-metallic metals. In my experience, it also tends to foul about like copper does. Fortunately the same cleaners that remove copper will also take out brass. The specific gravity is lower than both copper and bronze, but not by a great deal.
Steel is the most interesting material for bullets, in my opinion. Its biggest advantage, as I see it, lies in the fact that it does not foul. And if it does start to foul, it may be a “self healing” type fouling. Steel bullets are the hardest on barrels, and experienced shooters suggest they be fired only in chrome-moly barrels. I would agree with this recommendation too. The leadloy-type steel (12L14) used in these bullets is very machineable. It also holds tight tolerances well, probably better than the other materials. Steel is quite a bit cheaper than our other bullet materials. But long term storage may be a concern due to potential rusting. Steel has the lowest specific gravity, which could be looked on as a mixed blessing.
If you read articles like I sometimes do, skipping most of the technical talk the first time through, and jump to the end to see what the conclusions are, I won’t disappoint you. I found that there wasn’t really much practical difference in the ballistics of the copper, bronze, and brass bullets. Because the weight of each decreased in the order just listed, they could be driven faster as they got lighter. This characteristic made up for some of the decrease in ballistic coefficient as related to wind drift. There was only about 1″ more drift at 1000 yards for the brass bullet compared to the higher BC copper bullet. The brass bullet actually dropped about 10.7 inches less. Drop is not important though, if we’re zeroing the scope at 1000 yards. From a barrel-fouling point of view, bronze is probably the best. It’s the most expensive, too.
The bullet that shined in my eyes was the steel one. Because of its decreased weight, it can be pushed more than 150 fps faster than the copper bullet. This big velocity advantage almost makes up for the decrease in ballistic coefficient, the steel blowing just 2.4″ more at 1000 yards. The steel dropped 26.3″ less than the copper. That’s a fairly significant impact change of over 2 feet.
So what’s the best choice? Well, as with any rifle, the best choice is the bullet that is the most accurate at the range being fired. Perhaps, as the saying goes, all that glitters is not gold?
Comparison of Copper, Bronze, Brass & Steel 50 Caliber Bullets
Notes about the chart: For the 4 material types shown, the bullet shape stayed the same. It was a driving band-type bullet with a .502″ diameter body, 2.5″ overall length, 8 caliber tangent ogive, a .005″ diameter meplat, and a boattail .240″ long with an end diameter of .410″. Therefore the weight for each bullet changed because of differing material densities. Relative density is represented by the term specific gravity. The estimated velocity was calculated by an accurate internal ballistics program. The C1 ballistic coefficient was also computer-calculated from shape, specific gravity and velocity inputs. The same is true of the calculated barrel twist rate, resulting in a stability factor of 1.5 in each case. Wind drift (10 MPH) and drop figures, at 1000 yards, were computed by an external ballistics program and based on ballistic coefficient and velocity. All external ballistic coefficient and twist rate data is based on a standard sea-level atmosphere. Increasing the temperature or altitude of the gun will decrease drift, drop and required twist.
Comparison of Copper to Steel 50 Caliber Bullets
|weight||809 GR||703 GR (-13.1%)|
|velocity||2686 fps||2850 fps (+6.1%)|
|C1 BC||.992||.879 (-11.4%)|
|10 mph wind 1000 yd||39.39″||41.83″ (+6.2%)|
|drop 1000 yd||308.1″||281.8″ (-8.5%)|
This chart compares the differences between the two bullets with the greatest weight differences, made from copper and steel. It is interesting that the weight decreased by 13.1% with steel, but the ballistic coefficient decreased by 11.4% even though the form factor remained the same. This is due to the fact that ballistic coefficient increases with velocity. Since the steel bullet is lighter, it can be driven to higher velocities. The wind drift for steel is 6.2% greater, but drop is 8.5% less at 1000 yards.