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Discussion Starter #1 (Edited)
I've been there, drank the kool-aid and figured SD had a LOT to do with overall penetration of different caliber bullets. I've read those who say that a bullet with superior SD will outpenatrate a bigger bullet with lesser/lower SD. My experience did not always show me that though. I began to think "outside the box" and was impressed when I found how much better bigger bullets penetrated and killed than smaller ones (MY experience & opinion).

I recently read a very well done bullet penetration test that really opened my eyes. The test was on 10 different bullets for the .444 Marlin, a rifle I own and enjoy shooting. The test was very informative and enjoyable to read and see the bullets compared. However, there was also a "wild card" along, so to speak. A 180gr Barnes "X" bullet fired out of a 30/06.

Now, we all know how much better that a 180gr "X" bullet at 2700 FPS is going to penetrate than a lowly 265gr FTX from a .444 marlin at 2,000 FPS, right? Hold your horses there Buckoos! Eight out of ten of those .444 bullets (.429 dia) equaled or outperformed the 180gr 30cal. Holy Smokes! Well, it was right there in B&W and water (water jugs used for the penetration medium). For the record, only two of these bullets were cast ones.

To save you all from looking it up, the SD of the .30cal/180 is .271 The SD of the 265 & 270gr .444s is .205 and .210 respectively and the SD of a 300gr .444 is .232. Heresy, no doubt! How dare those lowly SD .429 bullets to outperform that studly 180gr Barnes "X" !!! :eek::eek:
 

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" Just how important is SD?....was impressed when I found how much better bigger bullets penetrated and killed than smaller ones (MY experience & opinion)."

Seems you answered your own question quite well. Sectional Density is a simple mathmatical relationship of weight and diameter. It meant a lot in black powder-lead bullet days as a way of predicting penetration. It even meant a little a little in early cup & core days but it hasn't meant anything useful since about 1950 or so. As a pure math calculation, it's easy for a .224 bullet to have the same SD as a .358 bullet but the penetration/impact sure won't be equal.

A .30 cal 150 gr. bullet has a SD of .226 no matter if it's a full metal jacket, hollow point, round nose, plastic tip, thin jacket or thick, partition or monolithic. They sure won't penetrate the same tho. As you have found, mass and cross-sectional area are important.
 

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Something to think about

I have not read the article but most people would agree a higher SD bullet will penetrate further. That is two 30 cal bullets at 2000ft/sec, the one with the highest SD will penetrate furthest.

However flesh, wood, wet pack, water jugs all effect bullets differently.

In Hatcher's notebook there is two pictures of a 30 06 penetrating wood. the first one penetrates about three times further than the second. The first picture is taken after the bullet has stabalized in the air, the second from about 50ft before the bullet is stable.

The second bullet hit the wood and tumbled through the wood causing it to penetrate much less.

The first bullet had fully stabalized and penetrated the wood very deeply. It would have been unstable in the wood but I think the wood prevented the first bullet from tumbling.

When bullets hit water if you have about 4Ft over your head you are safe. This is because the bullet is not spun hard enough to make it stable in water. and water does not offer enough resistance to stop a bullet tumbling.

6.625 x (VEL x BW/225120)/Cal^2 = Gel penetration. For your two bullets equals 84.7 and 150.8.

regards mik
 

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It entirely depends on how much the bullets expand. Once the bullet gets a bigger nose from expansion, the SD numbers change drastically. Some amount of the bullet's momentum is used up in deforming the nose. And, there may be loss of mass due to the bullet not retaining all it's weight.

That, generally, is what folks tend to miss about the concept. And it's why hard cast bullets penetrate all out of proportion to what we would expect..... they don't expand, and don't lose mass.
 

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If everything else is identical (velocity, bullet structures, caliber, metal alloys, target density and structure, expansion in the target medium, perhaps phases of the moon) then SD becomes very important in predicting penetration. If any of those things vary, SD becomes a poor predictor of very much.
 

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I think SD is still a good indicator, and useful for comparison as penetration potential. But it's best used when calibers, bullet construction, and velocities are somewhat near each other. By example, throw SD out the window if comparing a thin-jacket target HP with a Barnes TSX both with the same SD.
 

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As most have stated, SD is pure math with no regard to bullet construction. The construction (FMJ vs. HP varmit vs. Monothithic vs. A Frame or Partion) has a much bigger role in penetration.

Where SD comes in handy is when you are comparing bullets of similar construction and weight. For example, if I was comparing two different weight bullets of the same construction in the same caliber; the velocity and energy increase of a lighter weight .308 bullet may come at the cost of penetration from a higher SD bullet of same caliber but heavier.

Usually, I ignore the SD and look at the construction. If I feel my bullets aren't getting addequate penetration because of core seperation or too much expansion, I'd move to the same weight (therefore same SD) in a bonded or other expansion control construction. (Move from SST to Interbond or NBT to Accubond.)
 

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Discussion Starter #8
As most have stated, SD is pure math with no regard to bullet construction. The construction (FMJ vs. HP varmit vs. Monothithic vs. A Frame or Partion) has a much bigger role in penetration.

Where SD comes in handy is when you are comparing bullets of similar construction and weight. For example, if I was comparing two different weight bullets of the same construction in the same caliber; the velocity and energy increase of a lighter weight .308 bullet may come at the cost of penetration from a higher SD bullet of same caliber but heavier.

Usually, I ignore the SD and look at the construction. If I feel my bullets aren't getting addequate penetration because of core seperation or too much expansion, I'd move to the same weight (therefore same SD) in a bonded or other expansion control construction. (Move from SST to Interbond or NBT to Accubond.)
I am in agreement with you about bullet construction, totally. I'd suppose that's why I am/was so surprised at the results showing how 265, 270 and 300gr .429 jacketed bullets equaled or outperformed the .30cal 180gr "X" bullet, while being of jacketed construction and also of way lower SD. These bullets included the 265gr FTX bullet found in the LE .444 loads.

I would also most certainly agree that a .444 is not the best choice (nor even mine) for hunting at 200-300 yds, although it still packs plenty of punch at those ranges with proper loads. The trajectory, although better than the venerable 45-70, simply becomes too difficult to decipher at extended ranges.

My point actually was simply to show that it can certainly hold it's own for moderate range hunting styles (as many hunters do see) and the surprising results shown against a very highly regarded .30 cal load. If you might consider the 30/06 as the "gold standard" for calibers (one many are compared to, to show their relative effectiveness) than you might also use the well respected 180gr load from it as a kind of bullet standard for larger game.

It would be ridiculous to expect any one to run out and trade their '06 on a .444 Marlin, no doubt. But, showing the relative performance of the big bore against what many hunters think is a very fine combination in the 30/06 and shooting a premium bullet, I think the .444 shows itself to most certainly be a very viable alternative for those whose hunting is most always within the 200 yard envelope. And, again, this is performance with jacketed bullets that also offer excellent expansion to go along with the obviously excellent penetration.

Simply a very interesting result, to me anyway. :D
 

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I suspect that the .44 cal bullet just don't expand as much. They may not fragment as much either, given that the impact speeds are typically lower. More retained bullet weight = more momentum = more penetration.

My guess on the matter.
 

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Discussion Starter #10 (Edited)
I suspect that the .44 cal bullet just don't expand as much. They may not fragment as much either, given that the impact speeds are typically lower. More retained bullet weight = more momentum = more penetration.

My guess on the matter.

Well, the surprising thing is/was that's not the case here. Some of the loads, jacketed bullets, expanded to .7" and also over .8" and a good bit bigger than the perfectly mushroomed .30 cal 180gr "X" bullet. Only two of the 8 .444 loads that pretty much equaled or surpassed the 30/06 load were cast bullets. One very good jacketed load actually out-penetrated a cast load. The best jacketed bullet maintained nearly 90% of it's weight too.

Here's the link to the full test and results. I found it very interesting reading, especially given the wild card '06 load included for comparison. This is what makes the test so eye opening to me. Many feel that 180gr "X" bullet to be an outstanding performer, penetration-wise (myself included). To equal or surpass it's performance is noteworthy to say the least, IMHO.

http://www.marlinowners.com/forums/index.php/topic,81274.0.html
 

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In John Taylor's book "African Rifles & Cartridges" he set great store by sectional density, though he called it "weight to diameter ratio". But he admitted that some of the "high velocity magnums" like the .375 H&H or .350 Rigby Magnum seemed to perform all out of proportion to what he would have expected. The .350 Rigby mag in particular surprised him since it's 225 grain bullet had poor sectional density compared to most African rifles.
I long ago concluded that with expanding bullets sectional density in itself means little or nothing and even with fmj's it is just one factor. Take any given bullet loaded to different velocities and you'll often find that the lower velocity will have the greater penetration.
 

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No idea, then. There have been bullet penetration tests that showed lower velocities with greater penetration.... so.... I dunno.
 

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No idea, then. There have been bullet penetration tests that showed lower velocities with greater penetration.... so.... I dunno.
I think that's the answer, Mike...in many instances, higher velocity at the point of entry, especially for an expanding frontal area, means less penetration. Take that same 180gr 30-caliber bullet, but make it a FMJ, and everything changes. It is my opinion that as smaller bullets expand, they do not do so evenly, thus becoming somewhat unstable as they pass through the target or failing to penetrate as well as they might have. I think one reason big, wide meplat bullets penetrate well is they are not typically expanding a great deal and tend to stay in a nice straight line, as the pass through.

I'm not trying to merge threads here, but there is one making the rounds right now about 6.5mm bullets and why people like them so much. I think these two questions kind of blur into one answer: Bullets with good sectional density penetrate well, especially when you don't drive them too fast.
 

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In Thee Olden Days, when all bullets that were jacketed were simple cup and core designs, the way to get more penetration was to increase the SD of the bullet you were using. Some of the very high SD bullets of those days were renowned for deep penetration, like the 160 gr 6.5, the 170 grain 7mm, etc. Given those bullets (and manufacturing of bullets that was nowhere near as good as it is today), the comparing of SD was at least a semi reliable way to judge penetration of a given caliber/load.
Not so today. With Partitions, bonded core bullets, and monolithics like the Barnes X and others, penetration is more of a function of bullet construction than anything else. However, habits die hard, and shooters are a conservative bunch, so, the use of SD to compare bullets still goes on.
 

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hence all things being equal why I will take a .358 bullet over a .338 bullet over a .308 bullet. Its about bullet construction.

shoot straight and shoot often.
GF
 

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Randy Garrett posted on this board long ago a result he still has available to read on his web site, here. The same Hornady 500 grain RN .458 solids fired at different velocities. The faster the MV, the less penetration. About all I can think is this was caused by the stabilization issue Mikmarandola mentioned from Hatcher's Notebook. Inadequate gyroscopic recovery from initial pitch and yaw before impact. It would be interesting to see what would happen at 200 yards. Clearly, from Hatcher's test, the 50 yard range used in the test at the Marlin forum is not adequate to achieve that, so all bets are off on the results, except at 50 yards.

As to SD itself, assuming the bullets did have time to stabilize attitude fully, as Mike suggested, you really need to use the frontal expanded area in making the SD calculation. But that's not just before or after the bullet had expanded, but at all points along the path during expansion, too. The bullet starts into the target at one SD and changes to a progressively smaller one as it expands and/or loses weight. A high velocity bullet will usually expand earlier, so its average SD inside the test medium may be lower than the average SD of one that has lower starting SD but that opens up more gradually. So, neither initial or final diameter is enough information. You would actually need to know an expansion vs. penetration depth and a weight loss function and to find and sum their integrals to get the average result.

Finally, I would be remiss if I failed to point out that ballistic coefficient is a function of SD. For a given nose and tail shape, the higher the SD, the higher the BC in direct proportion. So SD affects trajectory as well as terminal ballistics. The same nose form that modifies SD to arrive at actual BC determines frontal resistance in a fluid. So, even for FMJ and cast bullet shapes that don't expand or lose metal, I would expect final penetration to be closer to a function of BC than to one of SD alone. A boattail won't make a difference in liquid as it does in air, so you probably have to knock 10-15% off the BC of a boattail to get a number for comparison in liquid penetration that considers only the nose form. Duncan McPherson's book on bullet penetration has penetration functions for different shapes.

For expanding bullets, obviously, BC drops with expansion.
 

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"The same nose form that modifies SD to arrive at actual BC determines frontal resistance in a fluid."

Soo...in application, if we had two 150 gr. .30 cal bullets, one is a boat tail spire point the other is a flat faced cylinder (wad cutter) their SD would change to follow their BC?
 

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Discussion Starter #18
Randy Garrett posted on this board long ago a result he still has available to read on his web site, here. The same Hornady 500 grain RN .458 solids fired at different velocities. The faster the MV, the less penetration. About all I can think is this was caused by the stabilization issue Mikmarandola mentioned from Hatcher's Notebook. Inadequate gyroscopic recovery from initial pitch and yaw before impact. It would be interesting to see what would happen at 200 yards. Clearly, from Hatcher's test, the 50 yard range used in the test at the Marlin forum is not adequate to achieve that, so all bets are off on the results, except at 50 yards.

As to SD itself, assuming the bullets did have time to stabilize attitude fully, as Mike suggested, you really need to use the frontal expanded area in making the SD calculation. But that's not just before or after the bullet had expanded, but at all points along the path during expansion, too. The bullet starts into the target at one SD and changes to a progressively smaller one as it expands and/or loses weight. A high velocity bullet will usually expand earlier, so its average SD inside the test medium may be lower than the average SD of one that has lower starting SD but that opens up more gradually. So, neither initial or final diameter is enough information. You would actually need to know an expansion vs. penetration depth and a weight loss function and to find and sum their integrals to get the average result.

Finally, I would be remiss if I failed to point out that ballistic coefficient is a function of SD. For a given nose and tail shape, the higher the SD, the higher the BC in direct proportion. So SD affects trajectory as well as terminal ballistics. The same nose form that modifies SD to arrive at actual BC determines frontal resistance in a fluid. So, even for FMJ and cast bullet shapes that don't expand or lose metal, I would expect final penetration to be closer to a function of BC than to one of SD alone. A boattail won't make a difference in liquid as it does in air, so you probably have to knock 10-15% off the BC of a boattail to get a number for comparison in liquid penetration that considers only the nose form. Duncan McPherson's book on bullet penetration has penetration functions for different shapes.

For expanding bullets, obviously, BC drops with expansion.
So, after reading this posting four times, I'll just figure that static SD numbers don't mean squat, which is kinda what I was figurin'.
Thanks. :D
 

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Interesting! One will see a very different result of the potential of SD in an expanding vs non-expanding bullet design. As was said...SD goes out the window when dealing with jacked expanding bullets. The instant the bullet begins expansion, SD changes due to bullet design.
Now...SD in a hard cast alloy bullet will penetrate different..based on the nose design!
So....Is SD density really important? Maybe yes in a cast non-expanding design, but that even gets fuzzy if the meplat area is different.
Regards, James
 

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SD is not the end-all variable when discussing penetration, or terminal performance in general, but it is still a valuable point of reference. When comparing bullets of like design, but differing weights/calibers, sectional density is one of only a couple things you can look at to figure out how they might perform.

To say it has no value is to misunderstand the principles of physics that determine how effectively a projectile will penetrate; and big game bullets must penetrate to be effective. If we can agree that a 6 inch diameter, flat disc that weighs 200 grains will not penetrate well, and that a 3 foot long, paper-clip thin, rod, of the same weight, also will not penetrate well, then it becomes evident that what WILL penetrate is an object with a shape somewhere between those two extremes. A round ball does not penetrate as well as a pointed projectile and neither does a wad-cutter. For anything less than approximately 35 caliber, pointed bullets are what you will most likely be using, and with them, SD is a variable worth knowing.

What we ask a proper big game bullet to do is pretty demanding. Even with all of the latest advances in bonded cores and monolithic bullet designs, the sectional density of a given bullet is still a valuable piece of information that does offer insight into how well it will penetrate. At least with respect to small-to-medium caliber, pointed, jacketed bullets, SD should not be dismissed entirely.
 
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