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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Stability: Fine Points to be Aware Of
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<blockquote data-quote="tim_w" data-source="post: 2634531" data-attributes="member: 11132"><p>So you do then agree that as a bullet is in flight the stability increases as velocity decays/over time? I want to confirm this as in your post (#3 of the thread) you seem to be attempting to refute Mikecr's statement to that very fact. This is the major issue I believe we were all having issue with. (Ref quotes below)</p><p></p><p></p><p></p><p>To which you replied with your examples of shooting your bullet here and further comments.</p><p></p><p></p><p>The part in bold is what we are in conflict with as was I am confident, the Hornady Ballictican you spoke to. No where in the rest of your post do you supply support for bullets loosing stability over TOF (time of flight).</p><p></p><p>You mention using JBM calculator and changing air density and seeing a corrlating drop in SG. I assume you are referring to the Modified Point Mass Trajectory calculator? First there are some accepted standards when it comes to gyroscopic stability factor or SG. Thru many real world controlled studies certain SG thresholds have been established. We only really need to reference two. If a properly cdesigned and constructed bullet has an accurately computed SG of 1.5 or higher for a traditional copper cup and lead core bullet it ensures full stability in all conditions. A mono copper or similar alloy bullet with a SG of 2.0 or higher will also be similarly stable in all conditions. I reference this as it can be used when interpreting the stability SG numbers in the JBM MPMT calculator's output..</p><p></p><p>Input whatever numbers you want for the bullet design or better yet just leave the defaults for this example. Include the point mass trajectory output and stability column by checking the last box on the left bottom just above the calculate button. Run the example. Look at the Trajectory column. Notice how with the increase in distance the SG number increases significant well above the 1.5 ideal threshold. Infact the default is ideal. The bullet has a SG 1.29 as it leaves the muzzle. By 100yd it's SG 1.42 (very close to tgat ideal 1.5) Jump out to 500yd and we have and SG 2.29. Way above the SG needed for not only full stability but maximum COF. </p><p></p><p> Now increase air density or decrease temp and allow the cal to adj to mimick the change in your live fire test from 60°F to 5°F. </p><p></p><p>Yes you will see a decrease in SG but look at the actual numbers as you increase distance specially 500yd as in your test. You will see SG at that distance/TOF the SG is way above the 1.5 threshold. Manipulate the numbers so at air density for 60°F has a SG of say 1.2-1.3 barely stabile. Look at the SG at 100 and 500 yds. You can see the bullet easily has a SG above 1.5 @ 500 yds. Stability always increases. A bullet can not be stabke enough to shoot bugholes at 100yds but hit sideways at 500yds Now adj temp to 5°F and see what the SG would be at 100 and 500 yds. You will see the bullet now clearly is below any SG that could afford stable flight. It may actually reach a SG @ 500 yd that could be stable if the bullet had not already reach a point of unrecoverable precession and yaw. No distance not 100 or 500 yds would ever produce a tight group with nice round holes. Basically an unrecoverable wobble.</p><p></p><p>As can be seen the JBM MPMT does not support but refutes your claim of stability decreasing with increased distance/TOF.</p><p></p><p> (Just using arbitrary numbers) Here is a hypothetical example of the ratio and relationship of twist rate velocity and bullet rpms looking at exiting the muzzle and at 1000yd:</p><p></p><p> A bullet from a 7.2 twist barrel with a MV of 3k fps and 300k rpm at a 1000 yd could have a hypothetical vel 1500 fps and 284k rpms. Vel decreasing 50% rotational rpm only 6.7% decrease. The V/RPM ratio went from 1:100 to 1:186. An 86% in ratio of vel to rpm. This would be the same stability at tge muzzle as if you you launched the same bullet at MV 1500fps from a 3.8 twist barrel. I think it's safe to say as long as a bullet is stable enough to print round holes at 100 yds it stablity will only increase from there and significant so the longer the TOF. </p><p></p><p>Addressing wound ballistics: This info was obtained from reference: <em>Conventional Warfare: Ballistics, Blasts, and Burn Injuries. Chapter 4 The Physics and Biophysics of Wound Ballistics. pg 107-118 </em></p><p><em></em></p><p><em>[URL unfurl="true"]https://medcoe.army.mil/borden-tb-conventional-warfare-ballistic-blast[/URL]</em></p><p></p><p>In terms of rpms or more correctly the SG as it pertains to wound ballistics and flight path of bullets thru different media what we are really speaking about is coefficient of drag or CD. CD is based upon the ratio of the velocity of the given projectile (Vp) to the velocity of the speed of sound (Vs) thru a given medium. (VP/Vs). That medium can be anything but usually (air, water, muscle tissue, various organ tissue etc). This is how you arrive at the "Mach" number . Below M1 is subsonic above it is supersonic for travel thru/in a given medium. There is the transition phase "transsonic". Transsonic is between M0.8-M1.20. Starting high and working down CD is stable just below and thru M2. When it reaches transsonic CD increases dramatically 3-4x!!! (M0.8-M1.2). Hence my earlier statement about subsonic load mv vel and the reason for increased twist rate for increased rotational vel unless mv velocity is lowered below that threshold. M0.6-M0.7 and lower CD is again very stable until, you get way down, around 100-115fps thru air.</p><p></p><p>As far as I know lung tissue has the lower density of major mass in the body. It's the tissue with the lowest retardation force and thus a bullet can achieve actual supersonic vel in it. The retarding force of other tissue had retarding forces that are tens of thousands of times greater than air. </p><p></p><p>The last part to effect CD and least critical is the stretchiness or more correctly the viscoelastic properties of the medium a bullet travels in.. As this is all about terminal wound ballistics we are referencing flesh. </p><p></p><p>As most offical live testing data was done with swine as its very similar to human muscle tissue we have these numbers. Muscle tissue CD 0.45.</p><p></p><p>This then brings us back to bullet stability. In this case stability in muscle tissue. Bullets of course have horrible stability in tissue. To put it into perspective for a bullet to have equal stability in tissue as it does in air even if you coukd make that tissue perfectly consistent it would require you to increase rpms at impact vel by roughly 32 fold. Going back to my previous hypothetical example 284k rpm would need to be increased to a tad over 9 Million RPM. Never going to happen thus bullet SG as long as its stable has the least effect. Bullet design is far more critical. Look at military bullets used and how they tend to yaw within a few inches of penetration. Increased total wound channels volume from fragmentation from the forced imparted from yaw vs traditional expansion.</p></blockquote><p></p>
[QUOTE="tim_w, post: 2634531, member: 11132"] So you do then agree that as a bullet is in flight the stability increases as velocity decays/over time? I want to confirm this as in your post (#3 of the thread) you seem to be attempting to refute Mikecr's statement to that very fact. This is the major issue I believe we were all having issue with. (Ref quotes below) To which you replied with your examples of shooting your bullet here and further comments. The part in bold is what we are in conflict with as was I am confident, the Hornady Ballictican you spoke to. No where in the rest of your post do you supply support for bullets loosing stability over TOF (time of flight). You mention using JBM calculator and changing air density and seeing a corrlating drop in SG. I assume you are referring to the Modified Point Mass Trajectory calculator? First there are some accepted standards when it comes to gyroscopic stability factor or SG. Thru many real world controlled studies certain SG thresholds have been established. We only really need to reference two. If a properly cdesigned and constructed bullet has an accurately computed SG of 1.5 or higher for a traditional copper cup and lead core bullet it ensures full stability in all conditions. A mono copper or similar alloy bullet with a SG of 2.0 or higher will also be similarly stable in all conditions. I reference this as it can be used when interpreting the stability SG numbers in the JBM MPMT calculator's output.. Input whatever numbers you want for the bullet design or better yet just leave the defaults for this example. Include the point mass trajectory output and stability column by checking the last box on the left bottom just above the calculate button. Run the example. Look at the Trajectory column. Notice how with the increase in distance the SG number increases significant well above the 1.5 ideal threshold. Infact the default is ideal. The bullet has a SG 1.29 as it leaves the muzzle. By 100yd it's SG 1.42 (very close to tgat ideal 1.5) Jump out to 500yd and we have and SG 2.29. Way above the SG needed for not only full stability but maximum COF. Now increase air density or decrease temp and allow the cal to adj to mimick the change in your live fire test from 60°F to 5°F. Yes you will see a decrease in SG but look at the actual numbers as you increase distance specially 500yd as in your test. You will see SG at that distance/TOF the SG is way above the 1.5 threshold. Manipulate the numbers so at air density for 60°F has a SG of say 1.2-1.3 barely stabile. Look at the SG at 100 and 500 yds. You can see the bullet easily has a SG above 1.5 @ 500 yds. Stability always increases. A bullet can not be stabke enough to shoot bugholes at 100yds but hit sideways at 500yds Now adj temp to 5°F and see what the SG would be at 100 and 500 yds. You will see the bullet now clearly is below any SG that could afford stable flight. It may actually reach a SG @ 500 yd that could be stable if the bullet had not already reach a point of unrecoverable precession and yaw. No distance not 100 or 500 yds would ever produce a tight group with nice round holes. Basically an unrecoverable wobble. As can be seen the JBM MPMT does not support but refutes your claim of stability decreasing with increased distance/TOF. (Just using arbitrary numbers) Here is a hypothetical example of the ratio and relationship of twist rate velocity and bullet rpms looking at exiting the muzzle and at 1000yd: A bullet from a 7.2 twist barrel with a MV of 3k fps and 300k rpm at a 1000 yd could have a hypothetical vel 1500 fps and 284k rpms. Vel decreasing 50% rotational rpm only 6.7% decrease. The V/RPM ratio went from 1:100 to 1:186. An 86% in ratio of vel to rpm. This would be the same stability at tge muzzle as if you you launched the same bullet at MV 1500fps from a 3.8 twist barrel. I think it's safe to say as long as a bullet is stable enough to print round holes at 100 yds it stablity will only increase from there and significant so the longer the TOF. Addressing wound ballistics: This info was obtained from reference: [I]Conventional Warfare: Ballistics, Blasts, and Burn Injuries. Chapter 4 The Physics and Biophysics of Wound Ballistics. pg 107-118 [URL unfurl="true"]https://medcoe.army.mil/borden-tb-conventional-warfare-ballistic-blast[/URL][/I] In terms of rpms or more correctly the SG as it pertains to wound ballistics and flight path of bullets thru different media what we are really speaking about is coefficient of drag or CD. CD is based upon the ratio of the velocity of the given projectile (Vp) to the velocity of the speed of sound (Vs) thru a given medium. (VP/Vs). That medium can be anything but usually (air, water, muscle tissue, various organ tissue etc). This is how you arrive at the "Mach" number . Below M1 is subsonic above it is supersonic for travel thru/in a given medium. There is the transition phase "transsonic". Transsonic is between M0.8-M1.20. Starting high and working down CD is stable just below and thru M2. When it reaches transsonic CD increases dramatically 3-4x!!! (M0.8-M1.2). Hence my earlier statement about subsonic load mv vel and the reason for increased twist rate for increased rotational vel unless mv velocity is lowered below that threshold. M0.6-M0.7 and lower CD is again very stable until, you get way down, around 100-115fps thru air. As far as I know lung tissue has the lower density of major mass in the body. It's the tissue with the lowest retardation force and thus a bullet can achieve actual supersonic vel in it. The retarding force of other tissue had retarding forces that are tens of thousands of times greater than air. The last part to effect CD and least critical is the stretchiness or more correctly the viscoelastic properties of the medium a bullet travels in.. As this is all about terminal wound ballistics we are referencing flesh. As most offical live testing data was done with swine as its very similar to human muscle tissue we have these numbers. Muscle tissue CD 0.45. This then brings us back to bullet stability. In this case stability in muscle tissue. Bullets of course have horrible stability in tissue. To put it into perspective for a bullet to have equal stability in tissue as it does in air even if you coukd make that tissue perfectly consistent it would require you to increase rpms at impact vel by roughly 32 fold. Going back to my previous hypothetical example 284k rpm would need to be increased to a tad over 9 Million RPM. Never going to happen thus bullet SG as long as its stable has the least effect. Bullet design is far more critical. Look at military bullets used and how they tend to yaw within a few inches of penetration. Increased total wound channels volume from fragmentation from the forced imparted from yaw vs traditional expansion. [/QUOTE]
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