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Old January 29, 2012, 01:56 AM   #1
xxxleafybugxxx
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Who's ready to explain some ballistcs to me?

To sum it up my question is why is a 9 millimeter penetrate better than a 45? The reason this is confusing to me is because I often compare the 7. 62 x 39 and the. 556
With the. 556, you have a smaller bullet moving faster and a larger bullet moving slower. However, the 7.62x39 is known to be a better penetrating round. On the handgun cartridge version however, you have the opposite
The smaller bullet moving faster (9mm) is the better penetrater. Why is this so? Shouldn't the same rule apply in both to cases? They clearly don't, so that's why im here asking you guys why ...
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Old January 29, 2012, 02:07 AM   #2
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The 9mm penetration issue relates to FMJ ammo only. Most 9mm JHP rounds penetrate roughly the same amount as .45 slugs do. The 9mm FMJ penetrates better by simple virtue of its added velocity as well as its smaller cross-section. With 5.56 it's a somewhat different matter, as the tiny 55-grain bullet is driven to velocities so high as to make the bullet unstable after impact. It will often tumble, break in half, or even fragment. The heaver 7.62 usually stays in one piece after impact and is able to keep going.

This is why the 5.56 is actually being recommended by some trainers as a home defense round these days, as the prior notion of a bullet capable of going right through the walls and into the neighbor's house has been proven mostly unfounded. Many 5.56 bullets actually travel less through drywall than standard handgun rounds.
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Old January 29, 2012, 02:34 AM   #3
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Weight is one way to get penetraton the other is velocity. If you drive a fmj 230gr 45acp 1150fps and a fmj 115gr 9mm 1150fps the 45acp should penetrate deeper. Hope this helps
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Old January 29, 2012, 04:48 AM   #4
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re:

Your question is actually more about physics than ballistics.

Consider the M193 round...55 grains at 3250 fps vs the original .45-70 "Cavalry" load...405 grains at 1250 fps.

There is little difference in the respective muzzle energies, but the big, massive slug will shoot through an Elk lengthwise, while the little .22 probably won't get reliably through the heavy chest muscles.

Mass and momentum cause the bullet to keep driving on when a resistance...an outside force...is encountered. Also remember that the faster the bullet is moving when it encounters a given resistance, the more rapidly it slows down.
Likewise, the less massive it is at a given impact velocity.

The 9mm round has less mass and less momentum, even though the velocity and energy are greater.
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Old January 29, 2012, 08:44 AM   #5
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The key to penetration is "Sectional Density" and bullet design. If it is a non-expanding bullet with the same Sectional Density, velocity and weight, bullet type is a major factor ... Pointed, flat, or meplat design. How a bullet tracks through real world animals and people is another factor, based on even more factors.

Since you cannot control the test data in self defense situations, clothing, size of opponent, distance, angle, etc... Not to worry, shoot well with good bullets, and it will sort itself out!!!

Nothing beats good shooting UNDER STRESS, to save your Butt ... targets don't mean squat on the streets ... UNDER STRESS, NO DO-OVERS ON THE STREETS!
Some of the best shooting cops on the streets fold like a lawn chair in a hurricane UNDER STRESS. So you can imagine what citizens will do who never shoot under real stress! More sweat in training means less blood loss on the streets!
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Old January 29, 2012, 09:17 AM   #6
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This issue is complex with no easy answers. All of the above responses can be correct but there is even more to consider. Bullets fired at rifle speeds react differently than when fired at much slower handgun speeds. Bullet construction is another consideration.

FMJ bullets fired at handgun speeds generally don't deform and tend to continue in a fairly straight line after striking flesh. Occasionally deflecting when striking bone. In order to help handgun bullets expand and cause more damage hollowpoint designs help with more rapid expansion, and reduce penetration. The smaller diameter, sharper point and faster speed of a 9mm pretty much offset the heavier weight of the 45. Especially since the 45 has a larger frontal area and a flatter profile. Both are pretty much equal in penetration in flesh. The 9mm penetrates barriers such as car doors and body armor better if both are in FMJ.

Rifle bullets fired at, or near 3000 fps react much differently. A 55 gr. 5,56, even in FMJ moving at 3200 fps, is going to deform much more than a FMJ 123 gr. 7.62X39 moving at only 2300 fps. The heavier, slower bullet will better penetrate in this case.

We still haven't considerd bullet construction or range. Some better designed bullets will hold together better at high impact speeds than others. In 1911 Tuners example, there are some 223 bullets that would come pretty darn close to equal penetration with the 45-70 example he used. Then there is range to consider. A bullet striking a target at 50 yards will react much differently than one that strikes a target at 500 yards after it has slowed considerably.
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Old January 29, 2012, 11:35 AM   #7
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We used to shoot old washing machines and crap scumbags left out in the AZ desert, and the 45 was always the worst performer. Had them actually bounce off some of the stuff we shot at. I think my 1100 ft/sec RWS pellet rifle has better penetration.
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Old January 29, 2012, 10:37 PM   #8
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There are two main factors that affect penetration. Momentum and frontal area.

Momentum is the product of mass and velocity. The more momentum (all else being equal) the more penetration.

Frontal area is how big the projectile looks from the front. The bigger the frontal area (all else being equal) the less penetration.

The problem is that when comparing the 9mm to the .45ACP, you're changing both factors at once, so the "all else being equal" qualifier can't be used.

The 9mm has less momentum than the .45ACP but also less frontal area. The combination of the two factors, one contributing to less penetration and the other to more. They actually cancel out fairly well in this particular case.

124gr FMJ 9mm and 230gr FMJ .45ACP penetrate roughly the same distance (to within an inch or two) in ballistic gelatin.
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Old January 30, 2012, 05:53 AM   #9
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re:

Quote:
124gr FMJ 9mm and 230gr FMJ .45ACP penetrate roughly the same distance (to within an inch or two) in ballistic gelatin.
This.

To take it a little further:

There are many variables that determine penetration, one of the main ones being sectional density. Between two bullets of equal mass and velocity, but different calibers, the longer bullet...the one with greater sectional density... will penetrate deeper, assuming similar nose shapes and bullet construction. i.e. Round Nose...Spire Point...Solid/Hollowpoint...Full jacket...Soft Point, etc.

And, of course, this also assumes a constant that's almost impossible to predict except under controlled conditions. The target itself, and anything that the bullet may encounter on its way to the target. Even a blade of grass will have some effect on the bullet's path.

So, the question of penetration on anything other than a repeatable test medium is largely theoretical. So, on the original question...there really isn't much of a difference.
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Old January 30, 2012, 10:23 AM   #10
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Quote:
The key to penetration is "Sectional Density" and bullet design. If it is a non-expanding bullet with the same Sectional Density, velocity and weight, bullet type is a major factor ... Pointed, flat, or meplat design
Bingo!
There's a good crude method of summing up the importance of section density.
Take a .30/06 and fire a 150 grain spire point bullet into a 100# sack of grain.
It won't go through.
Now take that same 150 grain spire point bullet and super glue it to an arrow shaft.
Use an 80# bow and shoot the same sack with that bullet tipeed arrow.
It will poke all the way through.

One is going well over ten times faster than the other, yet the slower one with a huge sectional density will penetrate more.
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Old January 30, 2012, 02:41 PM   #11
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It's because a 9mm is thinner and faster. A .223 is going a lot faster then a 7.62x39, but it's almost too thin and too fast which is why it usually fragments. An AK bullet is going 2.5x faster then a 45 and its a lot thinner. It's all about speed(velocity), weight(momentum), and sectional density(diameter). The .30 caliber bullets seem to be the ideal size for penetration, specifically the 357 Magnum. It's not too small like a .22(3), not too big like a .45 and it's going well over the speed of sound so it has enough energy to cut though. It seems like 9mm is good for barriers(doors, windshields) and the .45 is good for soft body targets(bad guys).
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Old January 30, 2012, 02:57 PM   #12
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Keeping in mind that both have roughly the same charge of powder behind them, it comes down to this which jmr40 summed up pretty well ...
Quote:
The smaller diameter, sharper point and faster speed of a 9mm pretty much offset the heavier weight of the 45. Especially since the 45 has a larger frontal area and a flatter profile.
The flatter round ball of the .45 isn't going to pierce as well as the long sharper parabola shaped point of the 9mm. Same charge in the .45 but a blunter nose trying to make a bigger hole.

edit:
removed reference to parabellum =parabola ... it doesn't, whoops.
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Old January 30, 2012, 05:03 PM   #13
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If you've ever looked at chronograph results for ammunition fired out of the same box - the velocities can vary significantly.

So obviously results of ballistics tests are going to vary.

But I would contend that even if you had 5 bullets that were identical and produced identical results going through the chrono - you'd still have different end results for penetration and expansion.

It's just the way the universe is...
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Old January 30, 2012, 11:30 PM   #14
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Quote:
...long sharper parabola shaped point of the 9mm (hence it's name parabellum).
"Parabellum" is Latin and means "for war".

"Parabola" is from Greek. The two words are unrelated.
Quote:
It's all about speed(velocity), weight(momentum), and sectional density(diameter).
Weight is not momentum, momentum is the product of velocity and mass. Weight is part of momentum, but it's not the whole story.

Sectional density is not diameter, it's the ratio of an object's mass to its cross-sectional area.

You can roughly assess the penetrative capability of a non-expanding round using momentum and frontal area or velocity and sectional density. Either pairing includes all the important variables, namely: velocity, mass and cross-sectional or frontal area.
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Old January 30, 2012, 11:39 PM   #15
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Weight is not momentum, momentum is the product of velocity and mass. Weight is part of momentum, but it's not the whole story.
And if you want to be really pedantic about it, mass =/= weight. Mass is an intrensic property of an object, whereas weight is the force exerted on that object by a gravitational field. On the moon, an object has the same mass as on earth, but not the same weight. An object moving in zero gravity still has momentum, even though it has no weight!
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Old January 31, 2012, 09:35 PM   #16
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And if you want to be really pedantic about it, mass =/= weight. Mass is an intrensic property of an object, whereas weight is the force exerted on that object by a gravitational field. On the moon, an object has the same mass as on earth, but not the same weight. An object moving in zero gravity still has momentum, even though it has no weight!
That's our smart boy. I look at it this way. The 7.62 is a solid pointy FMJ round at reasonable velocity and it plows through things. The 5.56 is a teeny round at goofy velocity that breaks up when it hits things. I once shot a gym sock full of sand at 100 yards with my pals AR and there was no exit wound. When I dumped out the sand I found little chunks of 5.56, the 7.62 AK I had blew it in two (after I finally hit it). The 9mm is a pointy FMJ round that punches neat little holes in things while the 45 is a lumbering blunt nosed fatso that knocks things down. Not very scientific but that's all I got.
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Old February 1, 2012, 01:02 AM   #17
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Well so much for trying to sound smart. 10 Beers summed it up nicely. The 9mm is like Bruce Lee and the 45 is like a linebacker. They can both beat the crap out of you but the small Asian one is a lot faster and thinner. On an unrelated note, they should really make an accurate 7.62x39 rifle if they haven't already. That round + accuracy = holy crap.
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Old February 1, 2012, 01:07 AM   #18
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CZ makes a bolt rifle chambered in 7.62x39 that is reportedly very accurate.
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Old February 1, 2012, 11:42 AM   #19
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Thanks for the correction John, good to know. So it's just an interesting coincidence in name similarities that the 9mm happens to have a parabolic shape to it (vs the spherical one of most RN bullets) and is termed parabellum but the two have nothing in common.
Guess I need to brush up on my Greek & Latin.

It's amazing the amount of 'smartness' flowing around on this sight when you consider how many people would think of all of us as just a bunch of dumb gun toting red necks.
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Old February 1, 2012, 11:50 AM   #20
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Warning.....off-topic Latin grammar alert.

"Parabellum" is Latin and means "for war."

Not quite correct. The word "parabellum" is an Anglicized mashup of the Latin clause para bellum, meaning "prepare for war" (para is the singular active imperative of the verb parare, "to prepare for," or "make ready for").
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Old February 1, 2012, 01:49 PM   #21
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You know, there are times it amazes me on the information you can get out of this site if you ask the right questions.
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Old February 1, 2012, 02:04 PM   #22
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Old February 1, 2012, 07:35 PM   #23
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The antebellum south was before the civil war, afterwards they called it the reconstruction era, not postbellum. So I guess you could say the war united the country, freed the slaves and killed latin.
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Old February 1, 2012, 07:52 PM   #24
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What, no mention of knock-down power?
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Old February 1, 2012, 10:34 PM   #25
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What, no mention of knock-down power?
No, thank goodness. In fact, I think Bart just universally settled the whole "9mm vs. .45" debate.
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