Ballistic coefficient

Lew, did you ever determine what the BC was for your 340I bullet?

If you have the sight settings from multiple ranges, shoot groups at a single range (same range, same point of aim different sight settings)and measure the difference in impact height between the different settings. Many free ballistics softwares provide for calculating a B.C. This way. Only other thing that is needed is muzzle velocity from a good chronograph. Other way to do this is to buy or rent a labradar. With less piddling you could produce a reasonable B.C. from a 5 shot string (keeping load and bullet constant) I would trust either of those methods above the bullet geometry calculator.

EdMehlig wrote:Lew, did you ever determine what the BC was for your 340I bullet?

I just now measured one, i came up with a BC of 0.267, Which is The same as the Lyman Gould

Idaholewis wrote:Well fellla’s I have been playing around trying to learn this BC Calculator This morning, As to how accurate it is? I really don’t know? It is not at all difficult to use, in fact I find it easy to use

This is the Calculator i used
http://tmtpages.com/calcbc/calcbc.htm#calculator

Here is what i am Getting with my BACO Money Bullet



Simply follow this Guide they have for the Lyman Snover Bullet


All i had to do was add the numbers from my Bullet above

The problem I have with this calculator is that if you put in the exact information above, but widen the the tip diameter to .4, it raises the BC to .679

On the other hand, take the same bullet parameters and lessen the tip diameter to .1, and it lowers the BC to .549

Same weird things happen by length as well; shorten bullet to 1” and BC is .702, lengthen bullet to 2” and BC drops to .586

That’s why I don’t trust this calculator for cast bullets, it seems contradictory to me (it doesn’t give weird results with Jacketed bullets, i.e. the longer the bullet - the higher the BC, regardless of weight)

There are only a few methods to determine BC with enough certainty to actually predict drifts and drops(all involve measuring actual flight of a bullet in some way) Commercially radar is the best method(measures distance and time). DIY methods utilize a chronograph, field measurements,and an exterior ballistics software. Method 3 utilizes multiple chronographs at known distances (Art Pjesa method). All can get you accurate drift/drop models. None involve only bullet geometry. BC values mean very little however if they are not ultimately utilized to predict flight characteristics. Lewis is right to be skeptical of the geometry calculator because flight characteristics can’t be predicted based on geometry alone

Idaholewis wrote:

McLoader wrote:Sorry just wanted to complete the thought above. These ballistic models are not above anyone and you can go to Gunwerks.com or a host of others to get really good and free software. The bullet geometry methodology is not exacting, and I would never trust those figures wholly without range verification(though they are likely close) but doing this homework can save a lot of headaches and money and really help you understand the performance and limitations of your equipment.

I agree, These BC Calculators will get you close, But you really need to test this stuff in the Field where it counts

Lewis, I agree with you and Jonathan....KISS! There is no reason to turn it in to rocket science.....It's a muzzleloader, not a sniper rifle!

I think the Confederates armed with Whitworth rifles may have begged to differ!

I'm betting they had no to clue to BC.

Nor did they need to....lessons of musketry taught in the years prior stand up just fine today. Without the internet......imagine(!)
Marksmanship can be learnt, with guidance, practice and application. You will not shoot a given projectile into tighter groups by knowing it’s ‘exact’ BC. That’s....the point I’d make.

For those who are interested, here is the response I received from Tom about the cast bullet estimator (http://tmtpages.com/calcbc/calcbc.htm#calculator) in question:

"Hello Josh,

Please forgive my tardy reply. Needed to get up more firewood while the
weather was cooperating.

The Ballistics Coefficient of a bullet is a product of two factors.
Bullet Profile (Form Factor)  and Bullet Sectional Density.

The aspect of the bullet's profile that has the most affect on the
bullet BC is the length of the radius that forms the curve of the bullet
ogive. A longer radius results in a larger initial BC value. Other
factors such as tip diameter, shoulder diameter and bullet grease
grooves then reduce that initial BC value.

A bullet with a wide meplate results in an ogive curve radius that is
longer than a bullet the same nose length and a smaller meplate

The change in the area of the flat nose has less effect on the bullet'
BC than does the change in the ogive curve radius. Resulting in the
counter-intuitive increase in bullet BC as the meplate diameter increases.

However, this effect is generally limited to a meplate diameter of less
that 1/2 that of the bullet diameter. A good estimate of a bullet BC
with a meplate diameter more than 1/2 of the body diameter is simply to
use the Sectional Density Value of the Bullet.

Sectional Density is equal to the bullet weight (in pounds), divided by
the bullet diameter (in inches) squared.

[bullet weight / 7000 / diameter / diameter] = Sectional Density.

Concerning the decrease of the cast bullet BC as the length of the
bullet increases. This decrease is attributed to the assumed increase in
cast bullet lube grooves creating more turbulence and drag on the bullet.

Checking the published BCs of older LYMAN  cast bullets with various
lengths of bullets with multiple lube grooves and the same nose profile
seems to bear out this assumption.

If using the newer no-groove coated bullets or bullets with micro groove
bodies, then probably a better estimate of the BC could be made using
the jacketed bullet estimator.

Hopefully this has made the concepts a little more clear for you.  Using
words to demonstrate mathmatical concepts is not one of  my stronger
abilities.

Ii you are so inclined, you can explore the math concepts in the help
file that accompanies the Cast Bullet Coefficient Estimator. Look in the
section titled "Calculation Results" for the breakdown.

Remember, a Bullet BC calculation value is intended as an initial
estimate that needs to be refined by range testing."


Respectfully
Tom Myers
tom1@wcta.net
https://www.tmtpages.com

Tank wrote:Nor did they need to....lessons of musketry taught in the years prior stand up just fine today.  Without the internet......imagine(!)
Marksmanship can be learnt, with guidance, practice and application.  You will not shoot a given projectile into tighter groups by knowing it’s ‘exact’ BC.  That’s....the point I’d make.

You are right! The bigger/smaller BC debate of the issue is a very linear and non productive form of thinking. The only purpose of the B.C. metric at all is to predict drift/drop values without actually having to develop them at each range. I will trade some figuring time at home for range time and resource intensive experimentation time. Both arrive at the same destination if taken to conclusion though so no one has to buy in to this. The question is how much range time do you get, and how much money do you want to spend to develop range settings for your rifle? For me the time spent with a ballistic calculator pays back in multiples of higher quality time behind the rifle and in understanding what it’s capabilities are. “We can do this the hard way or the harder way” is my perspective.

earlyg wrote:For those who are interested, here is the response I received from Tom about the cast bullet estimator (http://tmtpages.com/calcbc/calcbc.htm#calculator) in question:

"Hello Josh,

Please forgive my tardy reply. Needed to get up more firewood while the
weather was cooperating.

The Ballistics Coefficient of a bullet is a product of two factors.
Bullet Profile (Form Factor)  and Bullet Sectional Density.

The aspect of the bullet's profile that has the most affect on the
bullet BC is the length of the radius that forms the curve of the bullet
ogive. A longer radius results in a larger initial BC value. Other
factors such as tip diameter, shoulder diameter and bullet grease
grooves then reduce that initial BC value.

A bullet with a wide meplate results in an ogive curve radius that is
longer than a bullet the same nose length and a smaller meplate

The change in the area of the flat nose has less effect on the bullet'
BC than does the change in the ogive curve radius. Resulting in the
counter-intuitive increase in bullet BC as the meplate diameter increases.

However, this effect is generally limited to a meplate diameter of less
that 1/2 that of the bullet diameter. A good estimate of a bullet BC
with a meplate diameter more than 1/2 of the body diameter is simply to
use the Sectional Density Value of the Bullet.

Sectional Density is equal to the bullet weight (in pounds), divided by
the bullet diameter (in inches) squared.

[bullet weight / 7000 / diameter / diameter] = Sectional Density.

Concerning the decrease of the cast bullet BC as the length of the
bullet increases. This decrease is attributed to the assumed increase in
cast bullet lube grooves creating more turbulence and drag on the bullet.

Checking the published BCs of older LYMAN  cast bullets with various
lengths of bullets with multiple lube grooves and the same nose profile
seems to bear out this assumption.

If using the newer no-groove coated bullets or bullets with micro groove
bodies, then probably a better estimate of the BC could be made using
the jacketed bullet estimator.

Hopefully this has made the concepts a little more clear for you.  Using
words to demonstrate mathmatical concepts is not one of  my stronger
abilities.

Ii you are so inclined, you can explore the math concepts in the help
file that accompanies the Cast Bullet Coefficient Estimator. Look in the
section titled "Calculation Results" for the breakdown.

Remember, a Bullet BC calculation value is intended as an initial
estimate that needs to be refined by range testing."


Respectfully
Tom Myers
tom1@wcta.net
https://www.tmtpages.com

Thanks for posting this earlyg