Why would this amaze you?

Cat got your tongue?

Man asked you a simple yes/no question.

You wouldnt answer, instead asking why he wanted to know

You can be a piece of work.

SSR

Three posts up from where he asked the simple question (Is M/A = SD incorrect?), I used a green arrow and the word "mistake" in red capitals, pointing at Sd=m/A. I did this in two places.

Unless you attach a different meaning to "mistake", I would say that mistake = incorrect. Therefore, there must be another reason for asking the question "Is M/A = SD incorrect?"

Did he perhaps mean to ask "Why is M/A = SD incorrect?" Who knows. I cannot read minds, so I asked why he asked.

What is your problem?

http://forums.accuratereloadin...981023351#5981023351

Warrior

Seems to me that we have moved on from SD's importance and closing in on the conclusion that bullet shape and construction combined with its mass is what matters.

quote:

Originally posted by 900 SS:
Seems to me that we have moved on from SD's importance and closing in on the conclusion that bullet shape and construction combined with its mass is what matters.

And I definitely agree. But a bullet's shape is uniform and is therefore measurable. If it's measurable, this obviously means that you can put numbers to it.

I do believe it's certainly wrong to assume that the sectional densities we read in ballistics tables have much merit, but I have to believe that the principle of the mass per surface area is certainly valid. I just don't see how we can get past that.

someoldguy,
You are right. SD has little merit other than as a rough shorthand here and there. The principle of mass per surface area is a good start but only in combination with speed. In cahoots with a host of other factors, momentum applied to the surface area and the composition of the medium being penetrated, one can start making some assumptions.

I see rcamuglia has run out of answers and the cat has Cross L's tongue again.

quote:

I see rcamuglia has run out of answers and the cat has Cross L's tongue again.

Nope, always full of answers or something!

I've been playing on the Long Range Forum. I've been enjoying both of these threads, Momentum and SD, and have learned a lot from all of you. Basically we've come down to the minutia of the whole thing however still interesting.

I think I have gathered enough info for the time I've put in and have come to some conclusions that will serve me well in bullet choice. They are common sense conclusions derived from all the info:

Choose a bullet with a high BC

That bullet will have a high SD, it follows

That bullet will be "heavy for caliber"

Match the bullet's construction with the toughness of the game

Develop an accurate load with the bullet choice

Practice

Go "harvest"

For target shooting, just do the first 3 and practice.

As for the M/A question, the chart seems to be saying that it is SD. I know that SD equals the mass divided by diameter squared, but is there another way to express it as the chart seems to be doing?

.

quote:

Originally posted by ALF:
This is a most amazing thread!

We have at our disposal the opinions of the worlds foremost and imminent ballistics experts, their Doctoral theses and multitude of presented print all cite and expand on the concept of this ratio of a projectiles mass to it's representative or effective surface area!

They do not seem to have a problem with the concept of SD aa a dynamic enity.

Sir Isaac Newton who likely first explored and formulated a drag Equation ( allbeit with some error) had no problem with it.

One of the fathers of modern balisitcs Benjamin Robins had no problem with it, in fact he was likely one of the first to actually note it's effect in the penetration of the oak blocks he used in his pendulum tests! and Leonhard Euhler who translated Robin's book actually expanded on it ? .

We can go on, Each and every great written work in textbook form on the subject of wound ballisitcs or penetration mechanics , whether it's medical or forensic or for purposes of gun or munitions design when using classical mechanics as a model expand on the effect of mass and the representative surface area of the projectile.

So clearly I'm confused or say rather astounded by the declarations of the Experts on AR?

I deduce that M/A is, in effect, SD.

Obviously, changing the nose profile to a sleeker one while keeping mass the same will make the projectile longer than a projectile with a more blunt nose profile; the mass is still there and has to go somewhere if diameter remains the same.

A longer projectile with a sleeker nose as compared to a blunt cylinder slug of the same mass and diameter has a "dynamically greater SD" even though the diameter squared divided into the mass is equal?

quote:

A longer projectile with a sleeker nose as compared to a blunt cylinder slug of the same mass and diameter has a "dynamically greater SD" even though the diameter squared divided into the mass is equal?

That's the way I read it. The problem is with sleek pointy bullets is that they often don't stay sleek and pointed.
I wish I could find it now, but someone did a test somewhere on the web with a 100 grain .243 bullet at differing velocities. I think they got it down to around 1000-1500 fps so that the bullet wouldn't expand or lose mass. The result? A penetration depth of about 40 inches! (This could have been ballistic gelatin. I doubt whether it was wetpack.) A normally expanding .243 which usually loses a good amount of mass and expands to nearly twice the caliber penetrated to only about half that depth, or maybe a little less.

Having two different definitions of Sd is confusing to all of us. I suggest that we have two terms to SD and WSD.

SD = uses mass and cross sectional area

WSD = uses mass and wetted surface area

465H&H

Seems to me that we have run into Heisenburgs Uncertainty Principal. We can describe the projectile or its penetration but it changes as it penetrates so it is very difficult to asign canstants to a solvable equation.

I certainly am not versed in quantum physics ! but I quote

One striking aspect of the difference between classical and quantum physics is that whereas classical mechanics presupposes that exact simultaneous values can be assigned to all physical quantities, quantum mechanics denies this possibility, the prime example being the position and momentum of a particle. According to quantum mechanics, the more precisely the position (momentum) of a particle is given, the less precisely can one say what its momentum (position) is. This is (a simplistic and preliminary formulation of) the quantum mechanical uncertainty principle for position and momentum.

Stanford Encyclopedia of Philosophy

Sounds like thats where we are to me.

SSR

quote:

One striking aspect of the difference between classical and quantum physics is that whereas classical mechanics presupposes that exact simultaneous values can be assigned to all physical quantities, quantum mechanics denies this possibility, the prime example being the position and momentum of a particle. According to quantum mechanics, the more precisely the position (momentum) of a particle is given, the less precisely can one say what its momentum (position) is. This is (a simplistic and preliminary formulation of) the quantum mechanical uncertainty principle for position and momentum.

Not only that, but the Spurving bearings out of line with the Panometric fan can cause extreme side fumbling when introduced to the homogenious equation of continual wheelie cam momentum due to centrifugal force exerted on the central linear cam shaft.

Simple.

.

The pointy Berger "hunting bullet" is shaped to do what you say.

Construction needs help.

The secant ogive 6.5mm 130 grain Swift Scirocco meets the requirements of nose profile/SD for the effective ignorance of drag.

?


Is a bullet such as this, Perfect?

It has been said that making up science, as you go along, is not good.

Alf,
You have given the definition of SD as the weight divided by the diameter squared. You have admitted that this is a simple, imprecise ratio, that is used in a number of calculations.

You have also stated that one must not read more into SD than what it is. Now you say that SD is equal to M/A. This is not true and you mislead with this.

Above, you name drop a number of big names and you say "They do not seem to have a problem with the concept of SD aa a dynamic enity." This again is not true. All the eminent scientists you quote, recognise that SD is what it is, and that the dynamic aspect is something else.

SD can never be dynamic because it is a simple ratio, a fudge factor with which one calculates square bullets.

When there is talk about dynamic SD (now we know where Warrior gets this nonsense of dynamic SD and pristine SD from) and two forms of SD (SD and WSD), it is junk science. Witness rcamuglia's confusion: He thinks SD = M/A and that making a bullet longer for the same weight increases SD.

Cross L, I can assure you that any uncertainty does not apply to WhatThe, myself and a couple of others here. That it does apply to you, Alf, Warrior, 465H&H and rcamuglia, I am certain of. This is not meant as an affront to your character or a denting of your pride, it means you have not been given the right input and you are making decisions/assumptions based on wrong/insufficient data. Someone said, often you need to see the movie twice to get the point. It is also true that some never get the point, some do not want to get the point and some do not care about the point. To each his own.

Alf posted:

quote:

Our mistake is to take the formula of Sd = m/d^2 to be asbolute! Well is it? if it were then our projectile would be square!

You said it. It is an imprecise ratio, a shorthand method, that means nothing, and becomes something else as soon as motion is added.

Look at your post above. You make that statement about SD and then go on a treatise about spin rates, stability, BC and drag. All of these are dependent on weight (mass), shape, centre of gravity, speed, surface finish, length and a host of other factors. I do not see diameter squared in any of this and, according to your finding and reporting, that is half of what SD is.

Give it up man. Revisit the 5 year old Sectional Density thread and learn something.

quote:

Witness rcamuglia's confusion: He thinks SD = M/A and that making a bullet longer for the same weight increases SD.

I am a tad confused it might seem, but I understand the definition of Secitonal Density.

SD equals the weight of the projectile divided by the square of its diameter.

I do believe that SD has plenty to do with penetration when comparing projectiles of equal weight.

When the chart and posts of Alf and Warrior depict the change in nose profile of the equal weight projectiles and the affect this has on penetration, it tells me that "in effect" the SD of the projectile has increased simply by changing its shape.

The sleeker bullet may have penetration equal to that of a more blunt bullet that actually weighs more and has a higher sectional density from calculation.

True?

.

quote:

Altering nose shape can alter the effective SD of the projecile...

Thank you.

rcamuglia,

quote:

SD equals the weight of the projectile divided by the square of its diameter.

quote:

When the chart and posts of Alf and Warrior depict the change in nose profile of the equal weight projectiles and the affect this has on penetration, it tells me that "in effect" the SD of the projectile has increased simply by changing its shape.

Where in your first quote does it mention shape?

Alf,
She walks with flat heels and leaves no marks. She walks with stilettos and leaves marks. Her SD has not changed. Why does she leave marks on the floor with stilettos and not with flat heels?

quote:

if you up the effective surface area of the projectile, the resistance to penetration goes up.

In other words, if you change the shape.

quote:

If you up the mass the resistance to penetration goes down

Only if the bullet has velocity, in which case it is called momentum. You and Warrior may call it by it's junk science name (dynamic SD) but truth is, you can vary penetration depth by varying shape, speed or weight.

quote:

Altering nose shape can alter the effective SD of the projecile,

'Effective SD' - More junk science. There is no such thing as 'effective SD'.

quote:

the drag reduction achieved in dense targets by blunting the nose shape into a FN bullet does exactly this ! The good folks at the Scandia Labs in New Mexico have shown that the truncated cone FN generates the least resistance to penetration of all the nose shapes and thus when given a relative number ( Young's nose shape Number) it gets the most favourable number.

In other words, they vary penetration depth by varying the shape. SD stays the same.

quote:

In other words, they vary penetration depth by varying the shape. SD stays the same

Makes sense as well.

Penetration will be vary with construction as well with no change in SD. But the SD changes inside the medium and affects the penetration, so why is SD not "dynamic"

.

.

I would say that in target tumbling of fmj bullets (along with SD) is'nt very interesting when you choose hunting bullets. And if i remember correctly SD's correlation to performance of hunting bullets is the essence of this threads topic.

Exit holes from expanding bullets in the biggame I have shot has as far as i recall always been nice and round. Which tells me they usually dont tumble. Small game shot with pointy fmj bullets usually only show two small round holes as well.

quote:

Shape absolutely changes or rather has an effect on dynamic SD !

There is no such thing as "dynamic SD". Shape can only affect SD if you change the weight or the diameter. Those are the elements of SD - weight and diameter.

quote:

Because in actual penetration SD is dynamic and not static as it is in a bullet sitting on the table!

SD is always static. It is approximately the ratio of the weight of an object to the surface presented to the direction of motion. It cannot be dynamic because it is not a force.
From this page: http://www.thefreedictionary.com/dynamic this quote: dynamic [adj] 1. (Physics / General Physics) of or concerned with energy or forces that produce motion, as opposed to static.

SD can change but it cannot cause change because it is a ratio not a force.

quote:

Lets say we have a non deformable............ shape... redistribution of mass ..... Centre of gravity ..... motion ........ Point of pressure position relative to the CG ...... magnitude of the overturning moment ....... In terms of dynamics ....... overturn at a different depth within the target than it's former shape .... the Sd will change at every degree of change in yaw angle in the target.

All of which is caused by shape change, speed change, material strength, the forces that influence bullet behaviour and none of which is driven by SD. Why do you mention all this, apart from using it as padding?

quote:

So for the bullet that turns early we see massive early drag, if it turns late the drag occurs late !

Drag is determined by the properties of the medium being penetrated, the deformation of the bullet, should it happen, the shape of the bullet and the speed at which it encounters the medium. SD remains a ratio describing the static bullet, as in "This bullet has an SD of 0.300." "This bullet has a blue coating." "This bullet comes from a red box."

quote:

This is very important as to possible lethality ... The Balle D bullet had it's CG at 63% from the tip ... it took a relatively long time to turn .... the drag was massive ... this had it's CG at 55% ... a ratio of 3.53 ....... a large transverse moment of inertia ......... round nose FMj CG 54% from the tip ..... transverse moment of inertia 15.1 .... ratio of 3.80 ..... it penetrates quite deep and then tumbles .... drag to be the highest, in the vitals ... big difference RN solids target Solids .... RN a great improvemnt ... old Kynoch Shapes ...... all 30 cal or 5.56mm they all have the same SD .... however because of the differences in shape ... distribution of mass their in- target behaviour, penetration and distance to tumbling is different .... that is why their energy tramsmission to target profiles are different...... Shape matters !

I could not have said it better. Shape is the big dog in this fight. SD just yaps along on the sidelines.

quote:

I think the difference in viewpoit is some lookat SD as a static defined value

This is true. We accept the definition as it is and do not add any junk science to it.

quote:

wilst I dont care what the values are of the bullet on the table, the BC listed on the box is meaningless, the SD listed on the box is meaningless......

Agreed. Any factor in isolation is meaningless.

quote:

shoot the bullet and now BC takes on life

Absolutely. BC is dependent on speed and changes as speed changes. To give meaning to BC, the bullet has to be in motion.

quote:

shoot the bullet and SD takes on life !

Nonsense. SD is a fudge factor ratio that is the product of weight divided by diameter squared. Square bullets. Add speed to the weight and it is called momentum. Calculate the frontal area of the bullet correctly to get the cross section and then use that. Please do not call it momentum density as some would mistakenly call it.

quote:

you say dynamic SD is junk science?

Yes

quote:

Would you say that the Beat knuebeuhl, the late Karl Sellier, Bo Janzon and for that propagate junk science?

No, because they do/did not make up the idea of "dynamic SD". Those are your and Warrior's words and now you have confused a number of others into thinking this is so.

To me, saying sectional density is meaningless is exactly like saying torque or horsepower is meaningless.

A Dodge deisel truck or Indy car's motors have these two properties. They may have greater torque or horsepower than a Datsun pickup or a Smart Car, but to say it means nothing until they are in motion either trying to tow a 30,000 pound trailer or zoom around a track is like pointing out the big, stinky 900 pound gorilla in the same room to your friend.

quote:

To me, saying sectional density is meaningless is exactly like saying torque or horsepower is meaningless.

The correct analogy would be that torque and horsepower be equated to momentum and kinetic energy. SD would be the bore and stroke of a cylinder - the swept area. A small difference is that, using bore and stroke to calculate swept area, is precise, while weight divided by diameter squared, is a fudge factor.

And as like bullet construction and velocity ++ lessen SD's useability to predict bullet performance, turbochargers, valve configuration, fueltypes and so on messes with bore/stroke as a measure of engine performance.

I have been looking for a good analogy and there we have one.

.

quote:

So SD in terms of the penetration proccess is in fact Dynamic

It is not. The SD has only changed. The fact that SD changed over the time it took the bullet to impact and stop, does not make it dynamic. When a factor is dynamic, it causes change in related factors. SD does not cause the changes that occur. SD is changed by the events and forces that occur.

Therefore, the impact speed (dynamic) resulted in the bullet momentum (dynamic) being countered by the drag force (dynamic). The drag force exceeded the tensile strength (static) of the bullet, causing it to deform, thereby changing the SD (static).

Gerard; ok whatever you say ! you win !

All are good points and I understand both sides.

But coming full circle........

quote:

I have read numerous posts about poor or unexpected performance regarding their bullets and I am coming to the realization that some of us are unaware of the importance sectional density in guilded lead bullets.

What does Sectional Density mean to you?

Captain Finlander

The answer to me is that it is very important, whether you think it's static or dynamic.

From the charts Warrior has posted, no other conclusion can be arrived.

Take any rifle.
Take a specific lead core bullet available in different weights for that rifle.
The available bullets with the highest SD will be the heaviest and have the biggest BC than the rest.
When fired, they will ignore wind better
At impact, they will have greater momentum
They will retain a greater SD in the medium
They will out-penetrate the lighter, lower SD bullets in the line

quote:

Therefore, the impact speed (dynamic) resulted in the bullet momentum (dynamic) being countered by the drag force (dynamic). The drag force exceeded the tensile strength (static) of the bullet, causing it to deform, thereby changing the SD (static).

Tensile strength!

What is tensile strength?

quote:

Tensile strength is defined as a stress, which is measured as force per unit area.

Source: http://en.wikipedia.org/wiki/U...ate_tensile_strength

What is force?

From Newton's Second Law, it is mass times acceleration.

http://en.wikipedia.org/wiki/F...ewton.27s_second_law

So tensile strength could be rewritten as:

Mass x acceleration / area

Hint hint.

rcamuglia

I conclude different

Take any rifle.
Take a specific lead core bullet available in different weights for that rifle.
The longest bullet will be the heaviest and have the biggest BC than the rest.
When fired, they will ignore wind better
At impact, they will have greater momentum
They will retain more momentum per exposed area in the medium
They will out-penetrate the lighter, shorter bullets in the line

You just dont need to calculate the SD value to know this

quote:

Originally posted by someoldguy:

Mass x acceleration / area

Hint hint.

Back to momentum/resistance?

Thats my favourite!

rcamuglia,
I have asked this several times. Neither you nor Alf nor Warrior can come up with an answer. In the chart below, SD did not change. Why did the penetration depth change?

quote:

The available bullets with the highest SD will be the heaviest and have the biggest BC than the rest.

Not true. There are many examples such as this.
Sierra #2180 30 cal 220ge has an SD of .331 and BC of .310
Sierra #2160 30 cal 180gr has an SD of .271 and BC of .501

quote:

When fired, they will ignore wind better

Not true. From a 30-06, with a 15mph crosswind and at 250m

a Nosler Partition 180gr at 2820fps has 8" drift
a Nosler Partition 200gr at 2650fps has 8.8" drift
a Nosler Partition 220gr at 2500fps has 13.6" drift

quote:

At impact, they will have greater momentum

Not true. The bullets above will have:

a Nosler Partition 180gr at 2820fps has 59.3 Lb-F/S
a Nosler Partition 200gr at 2650fps has 61.7 Lb-F/S
a Nosler Partition 220gr at 2500fps has 58.7 Lb-F/S

quote:

They will retain a greater SD in the medium

Not true. Retained weight will be very similar with the heavier bullet shedding more weight.

quote:

They will out-penetrate the lighter, lower SD bullets in the line

Not true. Penetration depth is very similar. To get significant variance in penetration, the construction must be varied.

Thank you 900 SS for pointing that out to someoldguy. In any case, juggling the formulae around, does not change my statement, does it?

.