How Ballistic Coefficient Affects the Downrange Performance of Hunting Bullets
By Gary Zinn
One of the recent trends in rifle bullet design has been toward those with very high ballistic coefficients. I believe this has been largely driven by the desire for bullets that will perform well in long range target/match shooting, but it has spilled over into hunting bullets, too.
Therefore, the question arises whether very high BC bullets give any significant advantage in hunting applications. I have crunched a lot of numbers in pursuit of an answer to this question and here are the more interesting things I learned.
What is ballistic coefficient?
Chuck Hawks has provided a succinct explanation of ballistic coefficient in his article The Ballistic Coefficient of Rifle Bullets as follows:
"Ballistic Coefficient (BC) is basically a measure of how streamlined a bullet is; that is, how well it cuts through the air. Mathematically, it is the ratio of a bullet's sectional density to its coefficient of form. Ballistic Coefficient is essentially a measure of air drag. The higher the number the less drag and the more efficiently the bullet cuts through the air. For purposes of flying through the air efficiently, the bigger the BC number the better."
"BC is what determines trajectory and wind drift, other factors (velocity among them) being equal. BC changes with the shape of the bullet and the speed at which the bullet is traveling, while sectional density does not. Spitzer, which means pointed, is a more efficient shape than a round nose or a flat point. At the other end of the bullet, a boat tail (or tapered heel) reduces drag compared to a flat base. Both increase the BC of a bullet."
From the perspective of hunting bullet performance, we may expect high BC bullets to fly to a longer maximum point blank range (MPBR) than lower BC bullets of the same caliber and weight, when fired at the same muzzle velocity. This implies that high BC bullets have at least marginally longer effective hunting ranges.
We may also expect high BC bullets to maintain higher velocities and energy levels than low BC bullets at any given range. The importance of this to the hunter is that, other things equal, high BC bullets will have better terminal performance (penetration, bullet expansion and energy transfer) than low BC bullets, although the difference may not be significant.
There are well over a hundred viable cartridges that might be used for hunting varmints and game animals, so a comprehensive study of the effect of BC on hunting bullet performance across such a broad spectrum is impractical. To focus this article, I consulted The 10 Best Selling Centerfire Rifle Cartridges in the USA.
The top ten list includes nine of the most popular small bore centerfire cartridges suitable for hunting, plus one cartridge, the 7.62x39mm Soviet, which I excluded because I judge it to be poor for hunting Class 2 game. (See Are AR-15 Type Cartridges Good for Hunting Deer?)
The nine remaining cartridges fit into each caliber slot, .224 inch through .308 inch, except for .257 inch and 6.5mm, so I added the .25-06 Remington and 6.5 Creedmoor cartridges to represent these bore slots. In addition, the only top ten representative of the 7mm bore is the 7mm Remington Magnum, so I added the 7mm-08 Remington as a representative non-magnum 7mm cartridge.
With a dozen cartridges selected, I next focused on bullet weights, muzzle velocities and conventional (G1) ballistic coefficients. I evaluated a single typical bullet weight and muzzle velocity (from 24 inch barrels) for each cartridge, comparing the ballistic performance of two bullets with relatively low and high BCs.
I used the Shooters Calculator Point Blank Range program to calculate the MPBR of each load, which I rounded to the nearest 5 yard increment. I calculated +/- 1.5 inch MPBRs for varmint loads (.223 Remington and .22-250 Remington), +/- 3 inch MPBRs for all other loads. Then I used the Ballistic Trajectory program to calculate the energy and velocity of each load at the 5 yard increment nearest to MPBR, plus at 200 yards for the game hunting cartridges.
I consider bullet energy at the point of impact to be a key variable for big game hunting bullets, but it is much less important for varmint loads. A well designed varmint bullet will have enough energy to upset violently at any realistic distance downrange, so impact energy, as such, is not a concern when one is plinking varmints.
Velocity is the key variable in varmint bullet performance, because a higher muzzle velocity will drive a bullet of given weight on a flatter trajectory than will a lower MV. In addition, a high BC bullet will maintain higher velocity downrange longer than will a lower BC bullet of the same weight and diameter, which enhances flat trajectory and extends MPBR.
These general points imply that cartridges that generate high MV, loaded with high BC bullets, yield the flattest trajectories and longest MPBR ranges. This is desirable for varmint hunting, as maximizing effective range is a prime consideration. The double effect of high MV with high BC shows up in the ballistics of the .223 Remington and .22-250 Remington loads I evaluated. Here are the key numbers. (MPBR is for a +/- 1.5 inch bullet path.)
.223 Remington: 55 grain bullet at 3240 fps MV:
Percent difference: BC 50, MPBR 4.4, Velocity 13, Energy 27
.22-250 Remington: 55 grain bullet at 3680 fps MV:
Percent difference: BC 50, MPBR 6.0, Velocity 13, Energy 28
The data clearly show how the higher BC bullet outperforms the lower BC bullet at each muzzle velocity. Even more interesting is the performance advantage that the Nosler E-Tip bullet, fired at 3680 fps, has over the Sierra SMP bullet, fired at 3240 fps. The E-Tip, fired at high velocity from a .22-250 rifle, has a 40 yard (18%) longer MPBR and is flying 566 fps (25%) faster at 265 yards than is the Sierra bullet at 225 yards, fired from the .223 Rem. The bottom line is that both high ballistic coefficient and muzzle velocity are clear advantages in long range varmint hunting.
I hasten to add that this does not mean that the .223 Remington is a poor varmint cartridge. The .223 is totally adequate for shots out to 200 yards, or thereabouts, as well as being more pleasant and cheaper to shoot than the .22-250. Conversely, the varmint hunter who often takes shots of some 300 yards or more would likely prefer the .22-250.
Detailed listing and discussion of the specific results for all ten hunting cartridge and bullet sets is impractical in an article of this type. Instead, I will emphasize summary results that convey a general sense of how BC affects the ballistic performance of hunting bullets. This summary is based on ballistic data compilations like those shown above, which I am not including here for sake of brevity.
The difference in BC between the low and high BC bullet loads in the ten game hunting cartridges (.243 Winchester through .300 Winchester Magnum) averaged 46 percent. The range was from a 32 percent difference in BC for the .243 Winchester loads to 59 percent for the .308 Winchester loads. My takeaway from this is it is not difficult to find bullets that have substantially different BCs in the calibers covered in the analysis.
The smallest effect of low versus high BC bullets was in the differences in MPBR. This difference averaged a mere 4.3 percent overall (or 11.5 yards). Rounded to the nearest 5 yard increment (with +/- 3 inch MPBRs), all MPBR differences for low and high BC load pairs were either 10 or 15 yards. Yes, a higher BC bullet will extend the MPBR range of a cartridge, other things equal, but by only a few yards.
High BC bullets have a somewhat larger effect on velocity at MPBR distances. Across the ten cartridges, this difference averaged 7.6 percent (204 fps), ranging from 6 percent for the .243 Winchester pairing to 11 percent for the .30-30 Winchester loads. (The lower BC .30-30 bullet was a round nose, not a spitzer.) These percentage differences may not be eye popping, but they are significant because they have a multiplier effect on bullet energy downrange.
The average difference in energy at MPBR was 17.7 percent (283 ft. lbs.) between the low and high BC bullets in each pair of loads. The range was from 13 percent for the .243 Winchester loads to 24 percent for the .30-30 Winchester pair.
To me, downrange energy is the most significant standard ballistics variable for a big game hunting load of any given caliber, because it implies much about the terminal performance of the bullet. Generally, for a bullet of given diameter, weight and construction, the greater the energy at the point of impact, the deeper the bullet will penetrate and/or the more it will expand and the more energy it will transfer. Therefore, a bullet with greater energy will generally be more lethal than will be a lower energy bullet of the same diameter, weight, and construction.
However, energy values are not directly comparable among bullets of different diameters and weights, so another level of analysis is needed if we want to evaluate the relative lethality of different sizes and weights of bullets. Put another way, we need a secondary measure of bullet killing power that we can use to compare the downrange power of different cartridges.
Killing power of popular hunting cartridges
I believe that the Guns and Shooting Online Killing Power Formula is the best game in town if one wants to make general comparisons of the terminal power of bullets of different sizes and/or weights. The formula may not be perfect, but it uses relatively simple math calculations to yield useful indicative results.
Guns and Shooting Online Owner/Managing Editor Chuck Hawks developed this formula to calculate the killing power of hunting loads, using downrange impact energy, bullet sectional density and bullet cross-sectional area as the input variables. I will call the output variable of the formula "KPS" (Killing Power Score). For a given load, the formula is KPS at y yards = (Impact Energy at y yards) x (sectional density) x (cross-sectional area), or simply KPSy = Ey x (SD x A)
To cross-compare the game hunting cartridges and loads, I calculated KPS scores of all loads (each cartridge, with low and high BC bullets) at 200 yards, plus the KPS of each load at its +/- 3 inch MPBR yardage, rounded to the nearest 5 yards, denoted with a hashtag (#). The results separated the cartridges and loads evaluated into three groups.
Popular cartridges that are suitable for hunting deer and other Class 2 game (up to 300 pounds) start with the .30-30 Winchester and .243 Winchester. These have the lowest KPS scores, at 200 yards and MPBR range, of the ten cartridges evaluated; also, the .30-30 Winchester has the shortest MPBR ranges with either low or high BC bullets.
.30-30 Winchester: 150 grain bullet at 2390 fps MV (SD x A = .0168)
.243 Winchester: 95 grain bullet at 3100 fps MV (SD x A = .0107)
Chuck Hawks judges that a bullet should have a minimum KPS of 12.5 to have adequate power for most deer and other Class 2 game weighing about 150 pounds, or less. A minimum KPS of 15.0 is likely prudent for larger Class 2 animals. By these criteria, the .243 and .30-30 appear to be adequate deer cartridges out to their MPBR ranges, but should be confined to ranges generally under 200 yards when hunting larger Class 2 game. Using ammo loaded with high BC bullets enhances the power of these cartridges.
To get the best performance from a .30-30, use the Hornady factory load with 160 grain FTX bullets. With a BC .330 bullet, launched at 2400 fps MV, this load gets a 200 yard KPS of 23.4 and a 230 yard MPBR with a KPS of 21.8. This blows away conventional 150 and 170 grain flat point or round nose .30-30 loads.
The killing power of the .243 Winchester can be maximized by using the heaviest, highest BC hunting bullets available and driving them hard. For instance, the 105 grain Berger VLD Hunting bullet (BC .545), driven at 2900 fps MV, yields a 200 yard KPS of 18.1, with a KPS of 16.1 at a 290 yard MPBR. This would be a hand loading proposition, featuring the heaviest, highest BC .243 hunting bullet that I could identify. It would be worth the effort for anyone wishing to use a .243 Winchester for all-around Class 2 game hunting.
Next is a group of five versatile, non-magnum cartridges, with adequate range and power for efficient hunting of all Class 2 game.
.25-06 Remington: 120 grain bullet at 3030 fps MV (SD x A = .0135)
6.5 Creedmoor: 140 grain bullet at 2700 fps MV (SD x A = .0157)
.270 Winchester: 130 grain bullet at 3060 fps MV (SD x A = .0146)
7mm-08 Remington: 145 grain bullet at 2800 fps MV (SD x A = .0163)
.308 Winchester: 150 grain bullet at 2820 fps MV (SD x A = .0168)
The mildest of these cartridges, the .25-06 Remington, is definitely a step up in power from the .30-30 and .243, with an MPBR comparable to the .243. Even with relatively low BC bullets, all five cartridges have MPBRs in excess of 250 yards, with plenty of terminal power to cleanly drop any Class 2 animal, assuming a vital zone hit.
With high BC bullets of the weights indicated, the .270 Winchester, 7mm-08 Remington and .308 Winchester generate 200 yard KPS power that is on the cusp of that desirable for hunting Class 3 game (300-1500 pounds); this is generally a KPS of 30 to 32.
Anyone who sets out to hunt Class 3 game with one of these cartridges should use loads with high BC, heavy-for-caliber, controlled expansion bullets; e.g, 140-150 grain, 150-160 grain and 165-180 grain bullets for the .270, 7mm-08 and .308, respectively. I would suggest keeping shots at Class 3 animals within 200 yards with these loads.
The final three cartridges are the heavy hitters among the popular small bores. The 7mm Remington Magnum and .30-06 generate power at 200 yards and beyond that makes them potent for all Class 3 game, as proven in the field for decades, with KPS values well over 30-32. Meanwhile, the .300 Winchester Magnum, firing heavy bullets, has power that is likely sufficient for all but the very largest or most dangerous Class 4 animals and is, arguably, overkill for Class 2 game.
7mm Remington Magnum: 150 grain bullet at 3110 fps MV (SD x A = .0168)
.30-06 Springfield: 180 grain bullet at 2700 fps MV (SD x A = .0202)
.300 Winchester Magnum: 200 grain bullet at 2850 fps MV (SD x A = .0224)
I believe the 200 yard and MPBR distance KPS scores for these cartridges and loads speak for themselves. The only thing I would add is that any of them is, really, overkill for hunting Class 2 game. I say this even though I know the .30-06, particularly, has been used to take a great many deer and other Class 2 animals over its century-plus of use as a sporting cartridge.
Summary and conclusion
High BC varmint bullets flatten the trajectory and extend the MPBR of varmint cartridges, but by only a few yards, as the .223 and .22-250 Remington examples show. This is of little consequence if shots taken at varmints are within the normal MPBR range of the cartridge in question. If one is taking shots much beyond the usual MPBR of the cartridge, though, high BC bullets would be beneficial, because every little bit of range extension, without having to do much holdover, would help.
Using .243 and .30-30 Winchester loads with high BC bullets definitely helps increase the killing power (KPS) of these cartridges. This is important, because these two are the mildest of the very popular cartridges suitable for hunting all Class 2 game. Even a modest boost to their terminal power is a good thing.
The five cartridges I grouped together as mainline Class 2 cartridges (.25-06 Remington through .308 Winchester) will perform well on Class 2 size game with all suitably designed (rapid expansion without much fragmentation) bullets. Whether a bullet has a relatively low or high BC is of no great consequence. Conversely, loads with heavy, high-BC bullets would be a benefit if one wants to use a .270, 7mm-08, or .308 to hunt common Class 3 game, such as elk.
The .30-06, 7mm Remington Magnum and .300 Winchester Magnum are so powerful that any difference in range and terminal power that may relate to bullet BC is hardly important at all. I believe the hunter using any of these on Class 3 game (or, perhaps, Class 4 predators) should make load selection based on the bullet design that is most suited to the specific quarry, with only secondary attention to whether the bullet used has a low or high BC.
In cases where high BC bullets do not give a definite, needed boost to downrange performance, shooting expense generally favors using loads with common, lower BC bullets. Here are two examples.
Dedicated varmint hunters are high volume shooters who often reload. The 55 grain Sierra SMP and Nosler E-Tip bullets cited as low and high BC .224 bullets have MSRPs of $22.53 per 100 for the Sierra bullet, $26.45 per 50 for the E-Tip. This difference in bullet cost clearly favors using the Sierra bullet, unless one really needs the 10 to 15 yards of added MPBR that the Nosler bullet provides.
A .308 Winchester has been my main deer rifle for many years. I never take shots over 200 yards and most of the deer I have taken have been at under 100 yards. Common factory ammunition, using 150 grain jacketed soft point bullets (BCs about .31 for most common JSP spitzer, flat-base bullets) generally costs $20 to $25 per box of 20. By comparison, premium loads using high BC 150 grain bullets (e.g., Nosler E-Tip, Federal Trophy Copper, both BC = .469) cost about twice as much. Since I take my shots well within the MPBR range of the standard loads, with very dependable results, I have no incentive to use the much more expensive premium ammo in my deer rifle.
Makers and marketers of bullets and ammunition would like you to buy expensive premium ammo with high-BC bullets, or those same deluxe bullets for reloading. Whether you actually need such products for your hunting situation is worth considering.
Copyright 2018 by Gary Zinn and/or chuckhawks.com. All rights reserved.