Home Forums Hunting & Shooting Hunter's Campfire Ballistic Coefficient Question

Skinnier profile, less surface area for wind to act upon.

The simple explanation is that the heavier the bullet for a given caliber, the longer the bullet will be for that caliber.... longer is more stable and more wind resistant, all other things being equal- as long as the twist is sufficient to stabilize the length of the bullet.

Copper bullets of the exact profile will not have the exact same BC as a cup and core bullet of the same profile because they won't have the mass to maintain energy as long as the heavier bullet in flight or to fight wind as well- velocities and other conditions also being equal...

if you recall high school physics it is best to think of B.C. as the coefficient of friction (drag in the case of aerodynamics)...
probably the closest visual cue to B.C that I have found is the length/girth ratio.
so the larger the girth of the round the more "draggy" light projectiles will get when the projectile gets shorter...

What the gun likes.
Accuracy wise???
BC high versus low ???
I think alot of it has to do with more weight in the back 2/3rds of a bullet in the longer bearing surface areas based on overall design.

Alot of flat based bullets are more accurate than others with higher BC,s.
Not always the case though.

Gotta find out what a gun likes.
Really not what the shooter wants it to like.
I think alot of guys dont realize that, or have blinders on too it.

JMO.

Wind drift is ALL about time of flight. The higher the BC, the more velocity a given projectile will retain, retained velocity = reduced flight time.

The profile doesn't really matter. You can shoot a 6mm and 7mm projectile of the same BC at the same velocity and the wind drift will be the same.

You may be letting the concept of sectional density slip in with BC. sectional density is more related to weight within a given diameter right?

They're the same size but the texture of the skin is totally different. The BB has strings that catch the air to give it spin. The nerf has a porous surface that catches air and really slows it down. Momentum really comes into play with this comparison. The nerf has almost no momentum at all with it's low density. Mass plays a big part in BC and the nerf has very little of it compared to the BB

Sectional density IS a factor of the ballistic coeficient.

There's nothing magic about certain diameters, it's just that the .25s and .27s came into being during an era when higher velocities were desired and achieved by using lighter (and therefore having lower sectional densities) bullets just like Swamplord said.

At one point the .308 155gn Lapua Scenar was listed as a BC of .504. The BC of the old .243 Hornady A-Max is .500.

Run them both @ the same velocity and the trajectory and wind drift is equal. While you certainly need a larger vessel and more powder to get the .308/155 going the same speed as the .243/105, when the speeds match, so does the wind drift and drop.

Regarding certain projectile diameters having an inherently higher BC, in particular the 6.5 and 7MM, IMO, that's because those 2 diameters in particular were popularized in Europe where long, heavy-for-caliber round-nose projectiles and the corresponding faster twist-rates were the norm. 140's in the 6.5 and 160/175's in the 7 along with the twist-rates needed to stabilize them were en 'vogue long before pointy tips and long sweeping boat-tails

It seems to me that it would be theoretically possible to have two bullets, let’s say one at 143gr which is a copper bullet and one at 147gr which is a non copper bullet that have the same length and shape and have the same BC because aerodynamically they are the same. But because they have different densities and weights, the copper bullet would have a lower sectional density.

BC = M / (Cd x A)
BC = ballistic coefficient as used in physics and engineering
M = mass
C_d = drag coefficient
A = cross-sectional area

https://www.jbmballistics.com/ballistics/calculators/calculators.shtml

High BC's in respective calibers are a result of competition. There are certain calibers that were best suited for high BC bullets because a lot of their factory offerings came with the appropriate twist since inception, as jwp already stated.

Competitors build on those baseline calibers and the bullet manufacturers R&D accordingly.

Chamberings like the 6.5 Creed are a perfect example of this. The 6.5 was designed from day one with tight twists and standardized chamber dimensions. As a result it will always be an inherently accurate offering and that caliber will always have the most advanced bullet development.

It's no different than any other type of competition really. The options for building a 10 second car on a small block Chevy are going to be more widely available than building a 10 second car on a Toyota 3.5L.

Close, but not quite (in response to Jake). With ALL else being identical except composition, the bullet made of denser material would have the higher SD and therefore the higher BC also.


Edit: I should clarify...if the outer dimensions and shape of two bullets are identical but they are made of different materials, the one made of the denser material would [be heavier and therefore have higher Sectional Density, and] have the higher BC.

I think most would agree that two bullets with the same weight and different diameters would have the smaller diameter bullet yielding a higher BC.

I encountered an anomaly with the .224" and .243" diameter 55 gr ballistic tips. The larger diameter .243 has the higher BC according to Nosler. Perhaps Nosler made a mistake?

55 gr Nosler Ballistic tips:

.224" G1 .267
.243" G1 .278

In the most simple of terms think a rocket design in a wind tunnel.

You’re not trying to establish how much fuel it will take to get the rocket to where it’s going, or how fast it has to be at launch, you’re strictly trying to establish which one is less affected by wind/resistance.

The other problems are addressed elsewhere. BC just establishes how slippery the projectile is.

A couple of people have eluded to similar arguments and this makes sense to me. The idea that by design or tradition, some calibers gained popularity more for speed than longer range efficiency, would imply that typical bullet weight and therefore length and twist rates would be skewed lower within the potential weight ranges possible.

It’s a good rule of thumb but that’s about it.

Makes complete sense to me.

This is a picky point but although sectional density and BC go up together, I think they go up for different underlying reasons mathematically. Sectional density goes up because you're pushing more weight through the same diameter, hence a higher sectional density bullet would penetrate more for a given weight. On the other hand, BC goes up because you have a longer, more streamlined bullet for a given diameter, hence the better longer range aerodynamic efficiency. For all intent and purposes though, your statement rings true because for a given diameter, a heavier bullet would also be longer, given the same general shape and tail design.

There are no magic diameters. The shape and sectional density of the bullet directly determine the BC. High SDs make for high BCs. Sleek bullets with pointy tips, long ogives, and boat tails make for high BCs.

The issue arises when the bullet designers can't design a long, sleek bullet. The most common reasons for this are magazine length and twist rate. For example, the 7mm (.284) and .270 (.277) diameters are very close to each other, as are the 6.5mm (.264) and .25 (.257) diameters. But the twist rates chosen for the metric catridges are faster:

.270 twist rate: 1:10"
7mm Mauser twist rate: 1:220mm aka 1:8.66"

.25-06 twist rate: 1:10"
6.5mm Mauser twist rate: 1:200mm aka 1:7.87" or sometimes 1:220mm

Longer bullets are harder to stabilize at the same bullet weight, and boattail bullets are harder to stabilize than flat base. So the faster twist gives bullet designers the freedom to design higher BC bullets at the same SD/weight. This is why the bullet population for 6.5mm and to some degree 7mm are generally high-BC, and those for .270 and .25 are generally low BC. If you took the metric bullet and scaled it to the neighboring imperial bore diameter, it would not stabilize well in the standard twist rate.

If you build a fast-twist .25, you can then design high BC bullets for it. The problem is that since the rifle is a one-off, there are no customers for your bullets while there are millions of fast twist rifles in the neighbor bore.

Sometimes a cartridge maker will try to break this chicken and egg problem by standardizing a new cartridge with a faster twist, for example the .27 Nosler with a 1:8.5" twist (almost exactly what the Mauser was over a century ago). But inertia is hard to overcome.