A lead bullet with exactly the same shape and speed as a cup-and-core or mono bullet will have better sectional density, yet will probably perform worse due to deformation.

If bullets are constructed the same regarding shape and materials, their SD should be the same, irrespective of size. Then mass and speed comes into play.

I would hazard a guess that the shape is more important (for optimum performance ) than the SD
You have to hit whatever you are shooting at first ...

The shape and construction materials  determine the SD.

But !!!!!!!!!!!!!!!!!!! if that same musket ball was a smaller diameter and weighed the same it would penetrate even further.

If the ball penetrates fully, why change the shape to penetrate further than fully ?

Ok, but thats like saying a truck has good penetration because it goes through walls.

Not a relevant analogy.

The truck goes through the wall because it's momentum is able to break the wall.

Drive a truck into a cliff face at any speed and there is no penetration to speak of whatsoever.

But !!!!!!!!!!!!!!!!!!! if that same musket ball was a smaller diameter and weighed the same it would penetrate even further.

If the ball penetrates fully, why change the shape to penetrate further than fully ?

It changed first when rifling came into play. It changed again when bullets got copper jackets, and was probably adjusted when cordite and nitro cellulose powder

If the ball penetrates fully, why change the shape to penetrate further than fully ?

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If u can guarantee that the target will never be further, at a different angle and of same resistance material, then I agree we need not better through and through on that specific target.

In the era of the Brown Bess shoot throughs of more than one enemy soldier in close ranks was far from uncommon.
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Ok, but thats like saying a truck has good penetration because it goes through walls. You are comparing 500 gr object versus 165 gr object - even .223 55 gr goes right through a person.
Nothing in this statement is worth consideration.

You sound punchy. I wasn't comparing anything but rather responding to a comment on musket balls. What might be worth considering is that there was epic penetration at very moderate velocity with a low SD number that wouldn't meet the threshold you proposed in your original post. The take away from that consideration could be that SD in an of itself isn't the determining factor of anything.

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Yo!  Oaf, naaaa, just saying its not really worthy a statement because at 500 g, its gonna go through a lot of human even at musket speeds, its purely mass in motion. When we talking 150 g, we need to start considering things like SD as factors that can aid better decision making when using a bullet.
  It’s not the SD of the unfired bullet that influences penetration, it is the terminal sectional density (TSD) of the deformed/expanded bullet that matters.
Would you not agree ?
A 150 g .308 bullet shot at 150 g .308 velocities will deform more than a 180 g .308 bullet shot at 180 g .308 velocities. I would therefore deduct that the bullet with a higher SD would perform better.

Would you not agree ?
A 150 g .308 bullet shot at 150 g .308 velocities will deform more than a 180 g .308 bullet shot at 180 g .308 velocities. I would therefore deduct that the bullet with a higher SD would perform better.

ONLY way to solve this is by working out a proper test regime
Then going and testing

In my mind, bullet construction is more important than SD. An FMJ with low SD will still out penetrate an expanding bullet of high SD, so the maths gets too complex for us to make useful predictive models due to expansion and fragmentation. The best is comparing the bullets you want experimentally (ballistic gel or similar), and see if you get the result you want. By this I mean that if you shoot impala in thick bush, a 150gr low SD bullet from a 308 is way more effective at stopping the animal than a 150gr high SD bullet from 6.5 Creedmoore. Both will pass through, but the 308 makes a bigger hole, and dumps more energy, and the animal will go less distance before it hides under a bush a dies.

http://www.wainsworld.org/Guns/PDF/Expanded%20Rifle%20Bullet%20Sectional%20Density%20List.pdf

Lots of animals have been hunted over the years with "frangible" PMP bullets in the 130 to 180gr weights. Avoiding bone has given success and enough penetration for quick kills. As mentioned, SD is not the major factor for thin skinned animals. Elephant, Rhino, Buffalo and Hippo is where SD would factor. Modern brass bullets have made a big difference where penetration is really required.

In the era of the Brown Bess shoot throughs of more than one enemy soldier in close ranks was far from uncommon.
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Ok, but thats like saying a truck has good penetration because it goes through walls. You are comparing 500 gr object versus 165 gr object - even .223 55 gr goes right through a person.
Nothing in this statement is worth consideration.

You sound punchy. I wasn't comparing anything but rather responding to a comment on musket balls. What might be worth considering is that there was epic penetration at very moderate velocity with a low SD number that wouldn't meet the threshold you proposed in your original post. The take away from that consideration could be that SD in an of itself isn't the determining factor of anything.

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Yo!  Oaf, naaaa, just saying its not really worthy a statement because at 500 g, its gonna go through a lot of human even at musket speeds, its purely mass in motion. When we talking 150 g, we need to start considering things like SD as factors that can aid better decision making when using a bullet.
  It’s not the SD of the unfired bullet that influences penetration, it is the terminal sectional density (TSD) of the deformed/expanded bullet that matters.
Would you not agree ?
A 150 g .308 bullet shot at 150 g .308 velocities will deform more than a 180 g .308 bullet shot at 180 g .308 velocities. I would therefore deduct that the bullet with a higher SD would perform better.

You're seeing a point where I didn't make one. The variety of factors that influence how a projectile will perform in a target are large. SD is one of many and I'd be very surprised to find that it makes enough of a difference to make a difference except where other factors like calibre, bullet construction and shot placement are sub-optimal. I'd also hazard a guess that any number that you came up with as minimum threshold could be disproven empirically in real world testing based on the vast number of variable involved.

In my opinion as a professional hunting ignoramus with close to zero practical furred game experience in the last 20+ years I'd hazard a guess that this is of most interest to people using marginal calibres for whatever they are hunting or those shooting animals at long range.   

Predicting performance from data attributes is an infinite circular argument.

You have to test.

From your rifle, with your load.

Theoretically, the better SD / velocity / ES / BC / bullet construction etc., should perform better.

Testing will show what is true.

And the

Predicting performance from data attributes is an infinite circular argument.

You have to test.

From your rifle, with your load.

Theoretically, the better SD / velocity / ES / BC / bullet construction etc., should perform better.

Testing will show what is true.

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And then still no matter what the stats - predictions are if your rifle is crappy or the rifle just does not like your feed, it still will not shoot.
I recall on my .303 I had a load that was right, everything was right it was actually about 5 % off from Master Driepoots predictions and it would not shoot>
I ended up stripping the rifle - rebedding retightening, sanding, filing, all the tweeks to furniture and lugs.
Put same load back down barrel 10 mm group at 100 m from a old Lee Enfield.