Four Keys to Ballistic Performance Exposed

After reading my blog about the Best Whitetail-Pronghorn Cartridge, a reader asked this about cartridge/bullet performance:

When discussing BC, is there something special about the 6.5mm other than there are already bullets made in the 140gr+ range? If there was a 130gr .257 would it be the equal?

I have a 25-06 but for longer range shooting folks tend to step to either side of .257 and go with .243 or .264.

Could a heavier/longer bullet in .257 pull it closer to being accepted alongside the heavy 6mm and 6.5mm’s?

Thank You,

Christian

Most 25-caliber cartridges, from the short-lived 25WSSM on left to the 257 Wby. Mag. on right, suffer from a dearth of high B.C. bullets.

You're thinking correctly, Christian. Your 25-06 Rem. is a classic, excellent pronghorn/whitetail round, but with longer/heavier, high B.C. bullets, it could be even better. Here’s why:

The Four Keys to Ballistic Performance

With bullets long, sleek and sufficiently heavy, any caliber can match the trajectory performance of the 140-grain 6.5mms. We’re going to get “deep into the weeds” here, but what follows are the keys to understanding the great mysteries of rifle/cartridge/caliber performance. Once you “get” this, you’re in the perfect position for answering that perennial question: "which is the best round for huntinganything?" Ballistic performance hinges on these four things:

1. Bullet caliber (width or diameter.)

2. Bullet shape (form factor — the contours of the projectile.)

There are lots of good .257 bullets available to handloaders, but none with the extremely high B.C. ratings of many .264 bullets.

3. Bullet density (specific gravity of materials. A lead bullet has higher B.C. than aluminum.) 4. Muzzle velocity (launch speed.)The first three determine a projectile’s Ballistic Coefficient rating. The higher the B.C., the more efficiently the bullet flies. After that its just velocity. So, make a .257 bullet long enough (sleek, tapered nose and boat tail) and/or heavy enough (lead core vs. marshmallow core) and you can match the relatively high B.C. of most 140-gr. 6.5mms. I have some 115-gr. Berger VLD .257 bullets with a G1 B.C. rating of .466. The 120-gr. Berger Match BT Target .264 bullet rates G1 B.C. of .453. We can calculate trajectories for both. The 257 Wby. will drive the 115-gr. 3,300 fps. The 6.5 Creedmoor will push the 120-gr. 2,900 fps.

Trajectory Tables Tell the Tale

Here are calculated trajectory numbers for the 115-gr. .257 (B.C. .466) with 250-yard zero, 10 mph right angle wind:

Range VelocityImpact Drop ToF Energy Drift

0 3300 -1.5 0 0 2781 0

100 3075 1.94 1.87 0.1 2415 1

200 2866 1.68 7.43 0.2 2098 2.73

300 2668 -2.83 17.25 0.31 1818 5.72

400 2478 -12.25 31.98 0.42 1568 10.1

500 2297 -27.37 52.4 0.55 1347 16.03

600 2125 -49.09 79.43 0.68 1153 23.66

Here are numbers for the 120-gr. 6.5mm Creedmoor (B.C. .453,) same 250-yd zero and wind.

Range VelocityImpact Drop ToF Energy Drift

0 2900 -1.5 0 0 2241 0

100 2689 2.83 2.39 0.11 1927 1.12

200 2493 2.31 9.62 0.23 1656 3.23

300 2307 -3.83 22.48 0.35 1418 6.92

400 2129 -16.53 41.9 0.49 1208 12.35

500 1958 -36.94 69.03 0.63 1022 19.75

600 1797 -66.47 105.27 0.79 860 29.36

The 6.5 Creedmoor isn’t looking so magical now, is it? Lets see if the famous 140-grain Match Hybrid Target .264 Berger bullet with its higher B.C. of .618 will do better:

Range VelocityImpact Drop ToF Energy Drift

0 2700 -1.5 0 0 2266 0

100 2552 3.2 2.71 0.12 2025 0.98

200 2413 2.54 10.79 0.24 1810 2.64

300 2279 -4.12 24.86 0.37 1615 5.5

400 2149 -17.51 45.66 0.5 1436 9.66

500 2023 -38.45 74.02 0.65 1272 15.23

600 1902 -67.9 110.88 0.8 1125 22.34

Wow! Despite its much higher B.C. rating, the 140-gr. Berger gets beaten by the 257 in the drop and energy categories, but not wind drift.

With its significantly larger powder capacity, the 25-06 Rem. should outperform the much smaller 6.5 Creedmoor -- but no high B.C. bullets are made to enable this. Still, increased velocity can make up the difference.

Next, let's see what happens when we fling that 140-gr. Berger (still B.C. .618) 3,000 fps from a 264 Win. Mag?

Range Velocity Impact Drop ToF Energy Drift

0 3000 -1.5 0 0 2798 0

100 2841 2.41 2.21 0.11 25090.91

200 2692 1.98 8.76 0.21 2253 2.35

300 2548 -3.26 20.12 0.33 2018 4.83

400 2409 -13.88 36.87 0.45 1804 8.42

500 2275 -30.53 59.64 0.58 1609 13.21

600 2145 -53.91 89.14 0.71 1430 19.3

Well, well, well! Look what 300 fps more velocity does for the 140-grain 6.5mm bullet. It’s now beating the 115-grain .257. OK. Time for our final trajectory table. Let’s give your mythical 130-grain .257 bullet a B.C. of .618 to match the 140-grain 6.5mm and set it adrift with the identical muzzle speed of the 264 Win. Mag. above:

Range Velocity Impact Drop ToF Energy Drift

0 3000 -1.5 0 0 2598 0

100 2841 2.41 2.21 0.11 2330 0.91

200 2692 1.98 8.76 0.21 2092 2.35

300 2548 -3.26 20.12 0.33 1874 4.83

400 2409 -13.88 36.87 0.45 1675 8.42

500 2275 -30.53 59.64 0.58 1494 13.21

600 2145 -53.91 89.14 0.71 1328 19.3

Aha! Note that all the numbersremain the same except downrange kinetic energy. Because the .264 bullet is 10 grains heavier than the .257 bullet, it packs more energy. But velocity, impact, drop, drift, time-of-flight, wind drift -- all identical. Study these numbers and you begin to understand how increased velocity can compensate for a lower B.C. (less aerodynamically efficient bullet) and vice versa.

I know of no bullet maker building a .257 bullet with the weight, length and form factor to equal the highest B.C. .264 bullets. The 115-gr. VLD Berger shown on R. is rated B.C. .466. I'm hoping Hornady comes out soon with an ELD-X that exceeds that. We'll handload it and see if a 10" twist 25-06 Rem. or 257 Wby. can stabilize it.

Caveat: Ballistic calculations via computer software programs don’t necessarily match real world conditions perfectly. Any rifle/bullet/cartridge combination can slightly alter rated B.C. Engraving of the bullet adds drag. It’s balance (wobbling, nodding, etc.) can increase drag. Even flight speed changes B.C. The B.C. of a bullet at 3,300 fps can change when that projectile slows to 2,800 fps, for instance. Atmospheric pressure (sea level vs. 10,000 feet, low barometric pressure vs. high) also alters drag. In general, however, computer calculations come darn close to reality. So believe, but verify with real world testing.

How to Maximize the Four Keys to Ballistic Performance

Now that the four keys to ballistic performance have been explained, our takeaway should be the knowledge that ballistic performance depends on the combination of projectile B.C. and muzzle velocity. If B.C. and muzzle velocities are identical, drop, time-of-flight and wind deflection remain identical despite lighter or heavier bullets. Kinetic energy, however, varies based on bullet mass. With equal B.C. and starting velocities, the heavier bullets always retain more energy at all ranges.

To maximize downrange performance from any cartridge, then, choose the highest B.C. bullets in any weight category. Manufacturers are really cranking up B.C. these days. Hornady’s line of ELD-X and ELD Match bullets offer some of the highest B.C. ratings in the business — and they’re constructed for serious terminal performance, too.

Christian, your obvious question now is probably why long, heavy, high B.C. bullets aren’t offered in .257? My guess is because most 25-caliber rifles (250 Savage, 257 Roberts, 25-06 Remington) have been built with 10-inch twist barrels. This twist rate might not be quick enough to stabilize long, sleek .257 bullets. The 257 Weatherby Magnum, however, adds enough velocity that it might stabilize a 125- to 130-grain, high B.C. projectile. Were such bullets available, I’d try them in a 257 Wby. If that didn't work, I’d consider re-barreling the 257 Wby. or a 25-06 Rem. to an 8-inch twist. Fun experiment. But a whole bunch of shooters would have to pester manufacturers like Hornady before such a bullet would appear. The squeaky wheel gets greased in the shooter’s world, and right now the 6.5mms are squeaking loudest.

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