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Rifles, Reloading, Optics, Equipment
Reloading
Bullet stability
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<blockquote data-quote="Hugnot" data-source="post: 2824676" data-attributes="member: 115658"><p>The .284 Berger Hybrid is one long pointy bullet needing a fast twist rate.</p><p>Using the accepted Miller Sg calculation:</p><p></p><p>[ATTACH]464813[/ATTACH]</p><p></p><p>A 9 or 9.5 twist will not stabilize the 195 Berger Hibird at sea level at mild temperatures- it's marvelous G7 BC (comparative to standard) will be severely compromised due to bullet wobbling - like tip describing a circle as it plows thru air instead of slightly pointing up. Chances are that 1.645 long bullet profiles might occur on paper. MV of 2850 looks good for most big 7m rounds. Cavitation most likely thru water at much lower speeds. </p><p></p><p>From Berger:</p><p></p><p>"A Bullet's <a href="https://bergerbullets.com/nobsbc/what-is-a-bullet-bc/" target="_blank">Ballistic Coefficient</a> is comprised of 3 basic components: Weight, Diameter, and Form Factor. The weight and diameter of the bullet combine to determine the bullets sectional density; that's the amount of mass packed behind the frontal area of the bullet. Then form factor is what describes how streamlined the projectile is. A bullet with a long pointy nose and a boat tail will have less drag, and a lower form factor, than a blunt, flat based bullet. In particular, form factor is a number that relates the drag of a bullet to the drag of a standard such as G1 or G7. G7 form factors for long range bullets are from 0.95 (low drag) down to 0.88 (very low drag) for most typical long range bullets. The lowest drag projectiles made on lathes with very aggressive shapes, <a href="https://bergerbullets.com/product-category/bullets/?caliber=375-caliber" target="_blank">like our solid bullets</a>, can have G7 form factors as low as 0.80, but that's pretty rare."</p><p></p><p>Calculating the 195 .284 HiBird FF.</p><p></p><p>[ATTACH]464814[/ATTACH]</p><p></p><p> The Berger twist calculator gets into BC degradation with inadequate twist rates - but - using a barometer will provide actual barometric pressures instead of an altitude table for Sg determination. Bullet revs over expected TOF degrade very little like over 1000 yards with a TOF of 1 sec.</p><p></p><p>Miller sez 1.5 Sg for adequate stability. I have found the Miller Sg to be conservative, obtaining good accuracy with Sg values between 1.4 - 1.5 but minor BC degradation might occur.</p><p></p><p>Looks like an 8 twist would be best for the long pointy .284 Berger Hibird. Adequate Sg values of 1.5 plus with a 9 twist may be obtained at high altitude/low pressure & high temperatures like 27-inch Hg (possibly 5000 ft plus) & 95 F but Sg's will be inadequate for most applications.</p><p></p><p>I'm real happy with my creepy .280 Rem shooting stubby 139 H SST's killing deers and such up to 400 yards. I also use the 139 SST against rodents and have done well up to 600. My .280 R has a 9 twist barrel & I would not waste money on something that would not shoot good like the 195 Hibird in my 9 twist .280 Rem.</p></blockquote><p></p>
[QUOTE="Hugnot, post: 2824676, member: 115658"] The .284 Berger Hybrid is one long pointy bullet needing a fast twist rate. Using the accepted Miller Sg calculation: [ATTACH alt="Screenshot (671).png"]464813[/ATTACH] A 9 or 9.5 twist will not stabilize the 195 Berger Hibird at sea level at mild temperatures- it's marvelous G7 BC (comparative to standard) will be severely compromised due to bullet wobbling - like tip describing a circle as it plows thru air instead of slightly pointing up. Chances are that 1.645 long bullet profiles might occur on paper. MV of 2850 looks good for most big 7m rounds. Cavitation most likely thru water at much lower speeds. From Berger: "A Bullet's [URL='https://bergerbullets.com/nobsbc/what-is-a-bullet-bc/']Ballistic Coefficient[/URL] is comprised of 3 basic components: Weight, Diameter, and Form Factor. The weight and diameter of the bullet combine to determine the bullets sectional density; that's the amount of mass packed behind the frontal area of the bullet. Then form factor is what describes how streamlined the projectile is. A bullet with a long pointy nose and a boat tail will have less drag, and a lower form factor, than a blunt, flat based bullet. In particular, form factor is a number that relates the drag of a bullet to the drag of a standard such as G1 or G7. G7 form factors for long range bullets are from 0.95 (low drag) down to 0.88 (very low drag) for most typical long range bullets. The lowest drag projectiles made on lathes with very aggressive shapes, [URL='https://bergerbullets.com/product-category/bullets/?caliber=375-caliber']like our solid bullets[/URL], can have G7 form factors as low as 0.80, but that's pretty rare." Calculating the 195 .284 HiBird FF. [ATTACH]464814[/ATTACH] The Berger twist calculator gets into BC degradation with inadequate twist rates - but - using a barometer will provide actual barometric pressures instead of an altitude table for Sg determination. Bullet revs over expected TOF degrade very little like over 1000 yards with a TOF of 1 sec. Miller sez 1.5 Sg for adequate stability. I have found the Miller Sg to be conservative, obtaining good accuracy with Sg values between 1.4 - 1.5 but minor BC degradation might occur. Looks like an 8 twist would be best for the long pointy .284 Berger Hibird. Adequate Sg values of 1.5 plus with a 9 twist may be obtained at high altitude/low pressure & high temperatures like 27-inch Hg (possibly 5000 ft plus) & 95 F but Sg's will be inadequate for most applications. I'm real happy with my creepy .280 Rem shooting stubby 139 H SST's killing deers and such up to 400 yards. I also use the 139 SST against rodents and have done well up to 600. My .280 R has a 9 twist barrel & I would not waste money on something that would not shoot good like the 195 Hibird in my 9 twist .280 Rem. [/QUOTE]
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