Does any know the G7 Form Factor of the (.30 cal) 230gr. Atips?

[Mr.Knowit all]

Or does anyone know how to figure it with the available data? I was currious if it was close to .800...
Thank You.

 

[DaveTooley]

You might spend a little time looking for that number yourself. It's not hard to find. BTW you have the G1 and G7 numbers backwards.

https://www.hornady.com/bullets/a-tip-match#!/

 

[dcali]

Form factor can be calculated from BC:

form factor = weight / 7000 / caliber^2.0 / bc

 

[Mr.Knowit all]

Thank you

 

[DaveTooley]

You're welcome.

 

[Mr.Knowit all]

You might spend a little time looking for that number yourself. It's not hard to find. BTW you have the G1 and G7 numbers backwards.

https://www.hornady.com/bullets/a-tip-match#!/

I'm not the sharpest tool in the shed but according to Litz's book the ELDM 225gr have a G7 form Factor of .879 and I was just comparing the 230 Atips to them. I thought the 230s would have a lower G7 form factor than .879 I don't understand what is backwards??? Maybe I was a little ambiguous with my question or do I need to read Litz's books again?
Thanks for taking a minute to answer my question.
Cheers

 

[DaveTooley]

Not everyone's numbers are accurate. Hornady's are accurate.

 

[Mr.Knowit all]

Understood. Thank you

 

[Ballisticboy]

I think part of the problem is that you are asking about form factors where as the data on the Hornady site is BCs apparently causing some confusion. I could not see or find any information on form factors on the Hornady site. While the two are related they are not the same. The damoncali formula will give you the form factors from the stated BCs.
Based on the Hornady figure the form factor is about .837 for the G7 drag law.

 

[dcali]

A lower form factor isn’t generally going to result in the highest BC for a given bullet length. You have to balance weight, BC, and length in a sort of compromise. Low form factor bullets are light and skinny, which isn’t optimal for BC (again, given a bullet length).

 

[fyrewall]

I think part of the problem is that you are asking about form factors where as the data on the Hornady site is BCs apparently causing some confusion. I could not see or find any information on form factors on the Hornady site. While the two are related they are not the same. The damoncali formula will give you the form factors from the stated BCs.
Based on the Hornady figure the form factor is about .837 for the G7 drag law.

I get: ((230/7000) / .308^2))/ .414 = .839 . Essentially as good as .837, .414 is the G7 BC. The ((230/7000)/.308^2)) would be the sectional density.

A lower form factor isn’t generally going to result in the highest BC for a given bullet length. You have to balance weight, BC, and length in a sort of compromise. Low form factor bullets are light and skinny, which isn’t optimal for BC (again, given a bullet length).

Upon looking some line up of bullets of the same caliber I see the BC proportionally increase with weight or sectional density. Could I make an assumption that the form factors of these bullets are very similar? This gets into comparing two bullets having equal BC's but different form factors and SD's. Like BC = SD/FF, given an increase in SD and a increase (not desirable) in FF could a lighter bullet have the same BC as a heavier bullet and the lighter bullet realize an advantage because it could be driven faster? Again, looking at the line up of same make bullets and same diameters and all having G7 BC's I am unable to find a lighter weight bullet having the same BC as a heavier bullet - I'm guessing the FF's are similar. Possibly, if I compared different makes of bullets I might be able to find 2 bullets of the same diameter and the same G7 BC but having different SD's and FF's and make an assumption that the lighter bullet with the lower FF should be selected because it could be driven faster.

 

[dcali]

I get: ((230/7000) / .308^2))/ .414 = .839 . Essentially as good as .837, .414 is the G7 BC. The ((230/7000)/.308^2)) would be the sectional density.



Upon looking some line up of bullets of the same caliber I see the BC proportionally increase with weight or sectional density. Could I make an assumption that the form factors of these bullets are very similar? This gets into comparing two bullets having equal BC's but different form factors and SD's. Like BC = SD/FF, given an increase in SD and a increase (not desirable) in FF could a lighter bullet have the same BC as a heavier bullet and the lighter bullet realize an advantage because it could be driven faster? Again, looking at the line up of same make bullets and same diameters and all having G7 BC's I am unable to find a lighter weight bullet having the same BC as a heavier bullet - I'm guessing the FF's are similar. Possibly, if I compared different makes of bullets I might be able to find 2 bullets of the same diameter and the same G7 BC but having different SD's and FF's and make an assumption that the lighter bullet with the lower FF should be selected because it could be driven faster.

If you look at the equation relating BC to form factor, BC is always proportional to weight. If you take the same bullet and add or remove lead, you will see a linear relationship between BC and weight. BC is also proportional (inversely) to the "reference area" (the bullet's circular cross section) - this shows up as [caliber^2]. Smaller bullets have higher BC - but it goes up by the square of the caliber, so caliber matters a lot.

The only thing left is form factor. So yes, if you are seeing linear relationships between two bullets with respect to sectional density (weight/caliber^2 in this case), then the form factor is the same. Note that most people define sectional density as weight/area, which will be different, so you need to use weight per caliber squared, not the typical definition.

Where you see a difference is in tangent ogives vs aggressive secant ogives (or hybrids). The latter have skinny noses, and are necessarily lighter than the former. But they also have a lower form factor. Because of the lower form factor, secant ogive bullets can be more efficient, but give up BC to their tangent ogive brethren, who more than make it up in weight. This assumes bullets of equal length and optimal drag geometry for each bullet in general (nose length, boattail length, etc).

The way around this is to make the secant ogive bullet longer, which will increase its weight and BC, causing it to pull ahead of the tangent ogive. But the cost in increased length is lower stability and overall decreased accuracy. A low form factor gives you more BC per grain of bullet weight, but at a cost of increased length.

 

[fyrewall]

Thanks for the speedy response - learn something new every day

 

[Mr.Knowit all]

I thought that the G7 Form factor was-in Laymen terms basically "how efficient the BC is, so to speak...." The mass of an object can effect the BC but the G7 FF addresses the design effeciency...?
Anyone one care to put in their 2cents on that?

 

[dcali]

I thought that the G7 Form factor was-in Laymen terms basically "how efficient the BC is, so to speak...." The mass of an object can effect the BC but the G7 FF addresses the design effeciency...?
Anyone one care to put in their 2cents on that?

That's right. It's a fudge factor relating the shape of the bullet to it's overall drag performance. It's not really a useful number for shooters - BC and weight are generally available.

 

[JPeelen]

Sorry, but while I understand damoncali's view, I do not share it at all.
The BC merges two very different attributes of a bullet into one number: its sectional density (diameter, weight) and its aerodynamic quality. So the user never gets a feeling how good the bullet from an aerodynamic point of view is, because sectional density always gets in the way. Only when bullet diameters and weights are identical, BC really tells me something about how good the bullet design is.
On the other hand, the form factor immediately tells me the aerodynamic quality of a bullet. We have a common, very realistic standard for modern slender bullets, the G7 drag model. The i7 form factor makes it obvious how good a given bullet design in this respect is.

It goes without saying that the actual trajectory is a result of both, sectional density and aerodynamic quality, BUT ALSO muzzle velocity which often is very different for different bullets, totally independent of BC. Therefore, I much prefer to look at the computed trajectory based on these three numbers instead of only the BC.

We should keep in mind that the BC we know was invented by Siacci around 1890 as a tool of desperation, because computing trajectories was time-consuming in an overwhelming way we have not the slightest idea of today. Today we have the luxury of tiny hand-held computers being able to do much more precise calculations than the precision of our data. So we should in my opinion do away with antedeluvian calculation aids like BC and directly look at aerodynamic quality (form factor) separate from mechanical attributes (sectional density) of a bullet.

 

[Ballisticboy]

The problem with form factors is the same as the problems with BCs in that you are attempting to use the drag law for one shape to represent the drag law for another shape. Unless your bullet is exactly the same shape as the reference G7 shape you will never obtain an accurate representation of the drag law for your bullet as the shape of the drag law is unique for every projectile design. This is why sixty odd years ago the change was made to purpose made drag laws for every artillery projectile design where the problems with using BCs had become critical. Of course, in most cases, the error by using a slightly out drag law shape is going to be very small for rifle shooting but at long range with match ammunition it may be significant. So using the form factor for trajectory calculation is not really any better than using the BC.
That is why the industry change to trying to produce purpose drag laws is welcome, but, the drag law must be properly produced to be of any use and it is my opinion, based on experience, that the use of fixed head doppler radars for this purpose is not suitable.

 

[JPeelen]

I fully agree with you that the goal should be having each bullets individual drag law available. Alas, apart from Lapua publishing such data first in 2009 (last updated 2015), manufacturers hesitate to publish their results. Bryan Litz also keeps the "custom drag" data under the wraps of the Applied Ballistics family of software.

 

[Ballisticboy]

I fully agree with you that the goal should be having each bullets individual drag law available. Alas, apart from Lapua publishing such data first in 2009 (last updated 2015), manufacturers hesitate to publish their results. Bryan Litz also keeps the "custom drag" data under the wraps of the Applied Ballistics family of software.

Whilst I always applauded the Lapua work I was always a little suspicious of the results and how they were obtained since apparently every bullet was fired at up to Mach 5. Some of the results we got using a proper set of MV and tracking radars also did not agree very well with the published figures.

 

[dcali]

It’s important to note that the form factor is no less tied to a standard drag function than BC is. It’s just a fudge factor to make the math look nice.

I personally think we tend overstate the importance of custom drag curves, but sure, if they can be had (and are reliable), then by all means, we should use them. I wouldn’t lose sleep over it.