BC - SD - bullet weight a scientific answer.

[Ds J]

There are many variables to keep in mind.

The formula you search for can only work for a case with a certain ratio between case volume and calibre. Using the formula on a 45-70 or a 444 Marlin will give a different result than the same formula applied to a 22-250 or 30-378.

The penetrating name of the 6.5x55 came into being during the 1930's-1960's when the long bullets delivered good penetration. I am pretty sure that, if the available 303 or 30-06 ammunition had similar capabilites (pushing a long bullet at 2400-2600 fps), these cartridges would have had the same names for penetration. The 7x57 is closer to the 6.5.x55 when it comes to the ratio between case volume and calibre, and it also had "marvelous penetration".

The trick is probably long bullets at the correct speed for the given bullet material.

[Toxxyc]

The penetrating name of the 6.5x55 came into being during the 1930's-1960's when the long bullets delivered good penetration. I am pretty sure that, if the 303 or 30-06 had similar capabilites (pushing a long bullet at 2400-2600 fps), these cartridges would have had the same names for penetration.

This. The problem is people have discovered that when you launch a .30-caliber bullet with the same BC at the same velocities as what a 6.5mm bullet does, you drastically increase recoil and pressure. To obtain the same ballistics with a .30 caliber bullet as a 140gr 6.5mm bullet, you'd have to fire a 190gr .30-caliber bullet at the same velocities. With the current .30-caliber cartridges available, that produces almost double the recoil of the 6.5CM.

People go smaller because bullet tech has increased. That same bullet tech is being applied to the other calibers as well, which mean the old .30-06 is also improving. However, it's at the cost of recoil and additional powder use, etc, and people trade that off against the acceptable performance of the 6.5mm bullets.

But NEVER will a 6.5 Creedmoor be a better hunting rifle than, say, a .30-caliber rifle, keeping biltong hunters in mind. I just refuse to believe it. At all ranges you can do with a .30-cal the same and better as what you can do with a 6.5 CM, you're just going to have to deal with the recoil.

[treeman]

7x57 and 175gr conventional bullet
8mm and 196gr

You are relating to the BC and SD of the 6,5mm in a hunting scenario as SD does not feature in target shooting.

....
Surely if it had no SD it would not fly well - make it of Styrofoam and it could be perfectly shaped perfectly BC, but with out sectional density it would go no where fast. ???? Again - I am guessing but ??? me thinks BC can not exist with out SD in external ballistics ( and terminal)

[Toxxyc]

BC also takes the bullet weight into consideration, as it has a parameter in the formula that requires the bullet's density. A styrofoam bullet will have a terrible BC.

EDIT: Think people confuse BC and SD. SD refers to a bullet's "penetrating power". BC refers to it's ability to overcome air resistance in flight.

[driepootx]

Sectional density (a bullet's weight in pounds divided by its diameter squared) describes a bullet's length for its diameter: The higher the number, the longer thebullet. Generally speaking, the larger a bullet's sectional density, the deeper it will penetrate.

Momentum is the one that is related to the oomph that will keep the bullet moving. Weight x speed.

[Toxxyc]

Momentum is what drives the bullet. Air resistance is what's working against said momentum. Ballistic coefficient refers to how effectively the bullet will overcome that air resistance to keep that momentum going for as long as possible.

[zs6hdv]

BC is basically SD with a shape factor added.

So two projectiles could have the same SD but different BCs.

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[RevA]

SD/BC wise of the calibers I've used these seem to bring down the game most effectivly:
222/223 - works very well with 50grain
22-250 - 55 grain
243 - 87 vmax seems to be magic as is 80-90grain soft points
6.5 be it x55 or Creedmoor or 260 work best with 140 grains
270 - 130 grains get the best results
308 seems to work wonderfully with 150-168 grain bullets.
30-06 z loves 180 grain bullets
300 win mag - 180 grain is magic.

[Messor]

It's easy to read the numbers, but for most it's not easy to put them into context.

1: BC does not scale, Bryan explains clearly why.
2: Larger calibers requires less twist to stabilize in scale than smaller calibers.
3: Going by 1 and 2, you can shoot heavier bullets slower to achieve the same wind bucking abilities.

What does this mean in real life.

1: The smaller you go in caliber, the faster you need to shoot the bullet in order to gain the same wind bucking in scale, meaning you will need to shoot a 22cal faster than a 243 cal(6mm), a 243 cal faster than a 264(6.5)mm, and a 264 faster than a 308.

The direct complications that derives out of this is:
A: The faster bullet will generally have the flatter trajectory, since you need to shoot it faster, but your barrel life will suffer for it, that is the trade off.
B: Your heavier bullet can be shot slower, barrel life is increased, and required less of a twist, but the negatives are a not so good trajectory, and more recoil.

2: Besides BC, you need to shoot the bullet.
That requires 1, a suitable twist rate, meaning you cannot just go with sleek 160gr bullet in a 6.5, you won't stabilize them, and secondly you lose the ratio of powder capacity to bullet weight, meaning suddenly you cannot shoot the bullets fast enough for your purpose.

Remember now, most things accepted in the shooting world have arrived from empirical testing, meaning people have experimented with things that work, many of those did not possess the math skills to explain it like Bryan.
How to explain the 6.5 in layman's terms:
You cannot shoot a 100gr bullet, it won't have the required weight to length to form scale, hence your BC will suffer.
You cannot shoot a 180gr bullet, you don't have the powder capacity to shoot it at a usable speed, nor the throat design, nor the magazine length, nor the twist.
You shoot bullets in the 140gr class, because that is the best ratio of usable parameters.

Why a 6.5 then?

It's one of the better ratios according to rifle design and human tolerance.
Humans cannot tolerate a lot a recoil, recoil is not good for target shooting, so instead of using a 308 with 200gr bullets, people use a 6.5 with 140gr bullets, it gets the same job done ON TARGETS. These days, go follow the PRS equip in the states, people went to 6mm's, needs to be shot slightly faster, produces less recoil, more repeatable results, again, ON TARGETS.

If recoil was not an issue, meaning rigs did not have recoil, then nobody would shoot a 6.5 or 6mm, no way no how, they would push a heavy bullet hard. A 147gr ELD-M in 6.5 have a BC of .697, very good, a 225gr ELD-m from a 30 cal have a BC of .777 !!!

The perception problem:
Many people cannot separate target shooting vs hunting.
Their minds tell them since their setup are doing good on paper the cartridge is a suitable hunting cartridge as well, and that is nonsense, but if you understand all the above then no need to repeat it.

For hunting, the best cartridge for YOU, is the biggest one you can comfortably control.
For target shooting, the best cartridge for YOU ,is the one that makes you competitive in the specific discipline, taking BC, recoil and barrel life into account.

BC is just a number, hunters with experience DOES NOT need a BC calculator, they know what their equipment does, many generations before us hunted successfully, they knew the trajectory of their bullets, and knew how to compensate for wind.
BC is now on the forefront not because of hunting, but because of long range target shooting, where it's a requirement to be factored into ballistic applications and equipment adjustment.

Know what is what and then it's not that hard, just don't let it take the fun out of shooting.

[treeman]

Very well put Messor'
I found this particularly useful : and secondly you lose the ratio of powder capacity to bullet weight, meaning suddenly you cannot shoot the bullets fast enough for your purpose.

Remember now, most things accepted in the shooting world have arrived from empirical testing, meaning people have experimented with things that work, many of those did not possess the math skills to explain it like Bryan.
How to explain the 6.5 in layman's terms:
You cannot shoot a 100gr bullet, it won't have the required weight to length to form scale, hence your BC will suffer.
You cannot shoot a 180gr bullet, you don't have the powder capacity to shoot it at a usable speed, nor the throat design, nor the magazine length, nor the twist.
You shoot bullets in the 140gr class, because that is the best ratio of usable parameters.