The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Guns, Politics, Gunsmithing & Reloading Reloading The use of Sectional DensityGo | New | Find | Notify | Tools | Reply | |
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| One of Us |
Is there someone that can explain the use and need for Sectional Density of bullets. | ||
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Sectional density has no need on bullets it should be outlawed. | |||
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Do a search for "sectional" under the Find tab and you'll find all of the replies you could ever want. | |||
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Sectional density quantifies the penetrative ability of bullets - sorta. Briefly, a long thin bullet penetrates better than a short fat one of the same weight. Half a century ago when most all bullets were cup-and-core SD was useful in deciding how well a particular bullet might penetrate in an animal. But with today's high tech bullets the usefulness of SD is limited. . | |||
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+1 And The management at Woodleigh the first against the wall when the revolution happens. | |||
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If SD is meaningless, then will a 300 grn. .458 tsx penatrate just as well as a 300 grn .375 tsx at the same velosity (2400 fps)? If not, why? | |||
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Sd does not take into acount bullet construction or impact velocity, hence it is by itself a meaningless term. Since bullet construction is not accounted for, why not throw a 300 gr .458" solid into the mix? How will it penetrate compared to the 300 gr .375" tsx? Conversely you can take bullets of identical caliber and weight, hence identicle sd and construct them completely different. One very lightly constructed, one more heavily constructed, one a solid. All identical sd's, all very different performance. __________________________________________________ The AR series of rounds, ridding the world of 7mm rem mags, one gun at a time. | |||
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that should be ignored because it IS meaningless today. SD is a simple math relationship based entirely on mass and diameter. Meaning a thin jacket/soft core light game bullet which may 'explode' on contact has exactly the same SD as a solid core Barnes bullet of the same weight and diameter but the difference in penatration will be massive. SD doesn't account for speed at all so the impact velocities should be the same for whatever bullets we may be (honestly)trying to compare. | |||
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If I understand it correctly, SD is a mathematical calculation based on the diameter & weight of a projectile that was derived long ago when bullets were either lead or a combination of lead and copper. For example all .308 cal 180g bullets have the same SD regardless of their shape, construction or velocity. With a traditional cup/core bullets , in the same caliber, the only way to increase it's weight is to make it longer thus increasing it's weight and SD and therefore increasing it's ability to penetrate, SD does not take into consideration the material, shape, construction or velocity of the bullet. Do you believe that a 30 cal 180g roundnose traveling at 2400fps made in 1950 is going to penetrate as well as a 180g 30 cal Nosler partition, Swift A-frame, Barnes X or CEB also traveling at 2400fps ? Not Hardly , but SD theory would have you believe so. Personally, I believe it's an antiquated idea(theory) that has been completely debunked by modern technology and testing.I believe those hanging on to the SD theory are trying to mix yesteryears apples w/ modern oranges. Hope this helps. | |||
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Well, When I select a bullet for the job or game i plan to use it for. I consider this. How is it constructed Is It a A_frame, a solid, or some other design. Are we not talking about SD? the mass of the bullet. to kill an elephant I would use a bullet that that was constructed to kill an elephant not a bullet you would use on a gopher.. | |||
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Alf, a raw egg and a golf ball have near the same SD. Would you suggest that if we throw each at the same velocity at a mud bank they would penatrate the same? Or even nearly the same? You do understand that a 150 gr. .30 cal bullet has a given SD no matter the rest of it's criteria and the SD will be the same no matter the highly variable point configuration/BC, at any velocity and for any construction? The SD will be the same no matter the BC, and it would be the same even if it were loaded backwards, but the penatration would be vastly different. All making SD a meaningless value in today's world. | |||
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Trying to discount SD by talking only about terminal ballistics is hilarious. If I am not killing an animal what good is your view point? | |||
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WOW! What a collection of thoughts. Sectional density is nothing more than the Mass ahead of the bullet base. A given force (pressure) will accelerate the bullet inversley to it's mass (A=F/M). When a bullet strikes a target the momentum (MV) is impacted across an area equal to the bullet diameter. The higher. The higher the force the greatewr the penetration. It has nothing to do with bullet drag or other matters. When some people get ahold of a High Shcool Physics book they become dangerous. | |||
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You are confusing cause and effect. Penetration depends on the force applied. Other factors depending on the medium will modify it but the basic force depends on the ballstic coefficient. It is nothing more than the applied pressure (force/area). By your thought process it is the trees moving that makes the wind blow. | |||
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ALF, if you have a formula for a "dynamic" sectional density it would be good if you show it. At this point, the only SD formula I've ever seen is a simple single-step calculation relating the diameter to the mass; that makes SD a 'static' thing that remains the same if the bullet is lying on the floor or in flight. All of this is esoteric but SD was originally intended to suggest the potential for penatration in game. It worked very well for lead bullets in the black powder days and continued to be useful in the early days of jacketed bullets because the constrution was pretty much the same for all cup and core bullets, thus penetration remained somewhat predictable by SD. All that begain to change in the 30s when bullet makers - mostly Winchester and Remington - developed methods of controlling the rates of expansion by variations in jackets and cores. Bullet options really exploded in the 60s when custom makers began to develop vastly different bullet construction designs. So, today SD has become meaningless so far as predicting penetration and that was its only meaning to start with. When we look at a bullet maker's manual that shows the SD for a caliber and weight being the same even tho the bullets themselves vary from thin jacket hollow point, exposed point spitzer, round nose, FMJ, etc, it sorta gives us a hint that SD has no valid application for today's hunters. | |||
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I see two discussions here. One pertains to SD and "in flight" performance of the bullet. The other vein is dealing with terminal performance of the bullet "in flesh". Folks used to use SD as a means of assessing the potential performance of a bullet in an animal. SD may not be the best way to assess the performance of a bullet once it arrives at and enters the animal but it definitely is still an importance factor in exterior ballistics i.e. how and if the bullet will even get to the animal with the performance needed to get the job done. NRA Benefactor TSRA Life DRSS Brno ZP-149 45-120 NE | |||
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Ballisiic coefficient; Has nothing involved in drag, Ha everything to do with Ballistic Coefficient (as in bullet drift and drop). | |||
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I thought sectional density was how many folks lived or would fit in 640 acres.  Aim for the exit hole | |||
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Drag in flight is ballistic coefficent. Sectional density is based on - suprise! - density and cross sectional area, it has no relationship to ballistic coefficent; they are totally different games. | |||
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Many thanks for response on this matter. I now understand it beter. | |||
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Drag is the "Drag" of the bullet. Ballistic Coeffifient involves Drag and Sectional density. Drag is a measurement of the energy loss. The Sectional density determines how much velocity (momentum) is lost. Google these terms for a good discussion. | |||
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You are speaking of Ballistic coefficient, not sectional density. Do yourself (and others) a favor and Goolge it up and read. | |||
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Why don't we talk about how to use the metric stated in reloading manuals. You can use SD as a relative measure of how bullets of similar construction and similar materials will penetrate across calibers. For example: 180 grn .308 has an SD of .271 160 grn .284 has an SD of .283 150 grn .277 has an SD of .279 Therefore all three of these bullet weight/caliber combination will have similar penetration characteristics assuming similar construction and similar velocities. The 270 being in the middle, 7mm being a little bit better, the 30 caliber a little less. Some historical rules of thumb: SDs of .3 or greater excellent penetration SDs of .27 reasonably heavy for caliber and very good penetration SDs of .25 middle weight for caliber SDs of .226 light for caliber with relatively low penetration compared to other bullet weights in its caliber. Remember its relative; i.e. a .308 165 grn round nose solid will likely out penetrate a .308 180 grn ballistic tip because of bullet construction. Mike Never under estimate the internet community to use any opportunity to reply to a post with their personal rant about their tangentially related, single occurrence problem.  What I have learned on AR, since 2001: 1. The proper answer to: Where is the best place in town to get a steak dinner? is…You should go to Mel's Diner and get the fried chicken. 2. Big game animals can tell the difference between .015 of an inch in diameter, 15 grains of bullet weight, and 150 fps. 3. There is a difference in the performance of two identical projectiles launched at the same velocity if they came from different cartridges. 4. While a double rifle is the perfect DGR, every 375HH bolt gun needs to be modified to carry at least 5 down. 5. While a floor plate and detachable box magazine both use a mechanical latch, only the floor plate latch is reliable. Disregard the fact that every modern military rifle uses a detachable box magazine. 6. The Remington 700 is unreliable regardless of the fact it is the basis of the USMC M40 sniper rifle for 40+ years with no changes to the receiver or extractor and is the choice of more military and law enforcement sniper units than any other rifle. 7. PF actions are not suitable for a DGR and it is irrelevant that the M1, M14, M16, & AK47 which were designed for hunting men that can shoot back are all PF actions. 8. 95 deg F in Africa is different than 95 deg F in TX or CA and that is why you must worry about ammunition temperature in Africa (even though most safaris take place in winter) but not in TX or in CA. 9. The size of a ding in a gun's finish doesn't matter, what matters is whether it’s a safe ding or not. 10. 1 in a row is a trend, 2 in a row is statistically significant, and 3 in a row is an irrefutable fact. 11. Never buy a WSM or RCM cartridge for a safari rifle or your go to rifle in the USA because if they lose your ammo you can't find replacement ammo but don't worry 280 Rem, 338-06, 35 Whelen, and all Weatherby cartridges abound in Africa and back country stores. 12. A well hit animal can run 75 yds. in the open and suddenly drop with no initial blood trail, but the one I shot from 200 yds. away that ran 10 yds. and disappeared into a thicket and was not found was lost because the bullet penciled thru. I am 100% certain of this even though I have no physical evidence. 13. A 300 Win Mag is a 500 yard elk cartridge but a 308 Win is not a 300 yard elk cartridge even though the same bullet is travelling at the same velocity at those respective distances. | |||
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ALF, what is your background? Regards, AIU | |||
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Oh no! | |||
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Thank you Mike. For Hunters, that is all we need to know about SD. | |||
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ALF, you show the very simple calculation required to obtain SD; note there is no entry for velocity, drag or construction. Properly stabalized bullets do fly point on so the forward cross sectional area remains constant over the course of flight. Even tho the bullets are dropping rapidly at long ranges they do not fly sideways to maintain the original angle of departure; bullet holes in 1,000 yard targets are quite round, not oblong. The rest of us have been addressing how SD applies to bullets; we didn't mention nor do we care about esotoric discussions of whirling shell fragments, pointy steel recoil pads, etc. But, trust me, if you substitute a rubber knife for your steel one you'll get a better idea of how constuction changes things even tho the SD of the recoil remains constant. | |||
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The knife doesnt change SD it changes resistance the first few inches of penetration. A rubber knife less than steel. | |||
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Since all bullets cross-sections are circular it seems to me that comparing it with any other shape (like a knife blade) has no bearing whatsoever for any comparison. I like to think that it is a combination of caliber and weight and velocity that inflicts damage. When I try to explain it to a new shooter/hunter I prefer to use the "needle" analogy to illustrate that velocity as a prime component of a measure of energy does not describe the effectiveness of the bullet on game as well as caliber and sectional density. If you take a 180 grain cylindrical projectile (like a .308 diameter bullet) and a 180 grain sewing needle (which would be very long indeed) and send them at equal velocities they would have equal energy. However the needle going through the body of the animal would inflict little damage (a very tiny cylindrical wound channel) but the .308" diameter bullet will create a much larger diameter wound channel. So, energy alone does not describe wound damage, caliber alone does not describe wound damage, and sectional density alone does not describe wound damage. _________________________________ AR, where the hopeless, hysterical hypochondriacs of history become the nattering nabobs of negativisim. | |||
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The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Guns, Politics, Gunsmithing & Reloading Reloading The use of Sectional Density
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