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#### dla

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Imagine we have two hardcast bullets of the same profile, same diameter, differing only in weight and length. They are both launched so that they have identical momentum(e.g 155gr @1350fps and 180gr @1162fps). Which will penetrate further and why?

#### peng

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They would be exactly the same because they both hit a steel plate.

#### Glolt20-91

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Does anyone make a 155gr WFN? If not, this is a moot issue.

#### 1cm

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Imagine we have two hardcast bullets of the same profile, same diameter, differing only in weight and length. They are both launched so that they have identical momentum(e.g 155gr @1350fps and 180gr @1162fps). Which will penetrate further and why?
Penetrate what?

That is the unknown in your example.

#### dla

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Penetrate what?

That is the unknown in your example.
Both fired into identical medium. Lets say ballistic gelatin.

#### cowboy1964

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The one that delivers a greater portion of its momentum from it's mass rather than its velocity (the 180gr).

More velocity = more resistance and less penetration.

fredj338

#### fredj338

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Its called sectional density. In your example the 155 has a SD of 89, the 180 104. The lighter bullet going faster also slows faster in any medium. If you drove the lighter bullet even faster as a solid, it might match the heavier bullet By increasing momentum, but you have to push it quite a bit higher.

#### CDW4ME

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155gr @1350fps and 180gr @1162fps

Sounds like a 10mm load (In before anyone said anything about full power)
The 180 has potential to penetrate 32+ inches of gel, based on this test:
10mm Auto Self-Defense Ammo Ballistic Gel Tests - LuckyGunner.com Labs
180 Federal Trophy Bonded 32''+ penetration, no expansion, 1,227 fps

Bonus unsolicited thoughts on the 180 load:
If you are carrying for SD against a bear, reasonable choice.
Over penetrative for SD against human.

ETA the 155 would probably go 32+ inches of gel too based on a non-expanding HP with less velocity that did:
Handgun Self-Defense Ammunition - Ballistic Testing Data
Remington 155 HTP 32+ inches penetration, no expansion, 1075 fps

155 hard cast also over penetrative for SD against human attacker.

#### pAZ Ron

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Its called sectional density. In your example the 155 has a SD of 89, the 180 104. The lighter bullet going faster also slows faster in any medium. If you drove the lighter bullet even faster as a solid, it might match the heavier bullet By increasing momentum, but you have to push it quite a bit higher.
For the gold star 💥 as the only response directly answering the question!

#### cowboy1964

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Its called sectional density. In your example the 155 has a SD of 89, the 180 104. The lighter bullet going faster also slows faster in any medium. If you drove the lighter bullet even faster as a solid, it might match the heavier bullet By increasing momentum, but you have to push it quite a bit higher.
But the lighter bullet in the OP's scenario IS being driven faster, hence why the momentums are the same. But as we both explained, the extra resistance from the higher velocity means the heavier bullet penetrates more. It really is more about that than about the SDs since momentums are the same.

From a math perspective I think a more interesting question is: how much more momentum (velocity) would the lighter bullet need in order to overcome the added resistance due to its higher velocity?

#### RichardB

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Both fired into identical medium. Lets say ballistic gelatin.
Where is the gell test to support the posited theoretical conclusions or to raise other issues?

MarkCO

#### MarkCO

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From a math perspective I think a more interesting question is: how much more momentum (velocity) would the lighter bullet need in order to overcome the added resistance due to its higher velocity?
But momentum is not based only on velocity.

#### fredj338

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But the lighter bullet in the OP's scenario IS being driven faster, hence why the momentums are the same. But as we both explained, the extra resistance from the higher velocity means the heavier bullet penetrates more. It really is more about that than about the SDs since momentums are the same.

From a math perspective I think a more interesting question is: how much more momentum (velocity) would the lighter bullet need in order to overcome the added resistance due to its higher velocity?
Why i said you would have to increase momentum with higher vel. SD is just another metric showing the ability of a bullet to penetrate.

#### fredj338

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Where is the gell test to support the posited theoretical conclusions or to raise other issues?
So here is an example. The 165 has higher vel thus momentum, 181 v 167. Expansion is almost identical but the 180 is still higher SD. The diff on penetration is the addl momentum by raising vel.

 22.4" .49" 1101

 Hornady 180 gr XTP Custom 20.9" .50" 933

#### Matteo1371

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The longer bullet goes deeper... Hmm, there are so many sex jokes to choose from.

fredj338

#### peng

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Its called sectional density. In your example the 155 has a SD of 89, the 180 104.
1) How are you calculating this? I have not worked with sectional density much but I understand it is weight/diameter squared. For a 40 cal I get (155/7000)/.16 and (180/7000)/.16 or 138 and 160 ?

2) Is this calculation always done with the beginning diameter? Doesn't this neglect the bigger final mushroomed diameter (if any) and the lessened penetration it would give?

Thanks.

#### MarkCO

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2) Is this calculation always done with the beginning diameter? Doesn't this neglect the bigger final mushroomed diameter (if any) and the lessened penetration it would give?
Yes, and yes. It's simply comparative, not real world. It really is only defined at an impact velocity. Most LE trained "firearms experts" opposite of me use muzzle velocity. If my client is so inclined, pretty easy to get them excluded under Daubert for common and simple mistakes.

Real world would take impact velocity, solid mechanics, computational fluid dynamics and the path through everything except STP air.

It is complex to do. It took a few hours to convince a judge that my methodology combined with MRI data was valid to prove an entrance velocity. That included three barriers prior to the skin. Skin, muscle, fat, bone, marrow, connective tissue and the organs all have energy thresholds for surface penetration and then traveling through. As the velocity decreases and the diameter increases, those variables change. Tearing, coring, cutting, smashing again, all have different levels of energy use.

After the hard work is done, it's actually easy to add up all the contributions from entry until final rest to come up with the entrance velocity.

That said, it's rarely beneficial to do the time consuming work to prove an entrance velocity as usually, it is only needed to prove distance from shooter to the person hit with a bullet. It's also why I am very careful what cases I take. 😎

peng and pAZ Ron

#### fredj338

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1) How are you calculating this? I have not worked with sectional density much but I understand it is weight/diameter squared. For a 40 cal I get (155/7000)/.16 and (180/7000)/.16 or 138 and 160 ?

2) Is this calculation always done with the beginning diameter? Doesn't this neglect the bigger final mushroomed diameter (if any) and the lessened penetration it would give?

Thanks.
There is actually a program i use. SD changes as the bullet expands, weight/dia sq, the expanding bullet dia is getting larger, weight is the same. Why its really a metric for solids, not expanding bullets, but its still relative. Drive identical weight bullets the same vel, the larger expanding bullet penetrates less.

peng

#### fredj338

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The longer bullet goes deeper... Hmm, there are so many sex jokes to choose from.
But in the above example, no.

#### dla

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Muscle is 1.04grams/cc