SF and SG

remingtonman_25_06

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I was looking at these and noticed on strelok pro they use the SF for a stability factor. However, when you go to Bergers site, they use SG. Are these the same, or not? I understand you want a 1.5 to gell full stability, accuracy, and BC.
 

remingtonman_25_06

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Ok. Well I was playing around with some 6mm bullets from my 10 twist 240 WBY. Inparticular is the 87g VMAX since this is my coyote rifle. When I enter everything into strelok pro, I'm getting a SF of 1.31 and the berger site shows a 1.29 so close enough between the 2. But, that seems a little low for an 87g bullet, no? I've read where they are supposedly stable from a 12" twist, but if my 10" twist 240 spitting them out at 3400fps only shows an SF of 1.3 that doesnt seem right, does it? How can an 87g bullet be marginally stable from a 10"? If I go buy the SF numbers, then I'm limited to about the 70g NBT with an SF of 1.66 Even the 80g NBT only has an SF of 1.45 Just doesnt seem right to me. Unless I'm missing something?? Seems pretty stupid to me to only be limited to 70g plastic tipped bullets in a 240 WBY...
 
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User4302021

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You must have messed up your inputs in Strelok.

Strelok is giving to me a SF of 1.66 for an 87 grain Vmax at 3300 fps.

The Berger calculator is for BT bullets and doesn't work w/flat base.
 

remingtonman_25_06

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I'm double checking my numbers and I'm still coming up with an SF of 1.3

10" twist, 87g VMAX, bullet length is 1.053" measured base to tip with my calipers, 3400fps, 30 degrees, 500 ft above sea level.

How are you getting 1.66?
 

remingtonman_25_06

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Screenshot_20201128-141445_Strelok Pro.jpg
Screenshot_20201128-141242_Strelok Pro.jpg
 

User4302021

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Apparently there is a discrepency in Strelok between the G1 & G7 libraries with this bullet. The G1 number makes logical sense while the G7 does not...as you have pointed out.

My first number was at 60°F and 28 inHg. I adjusted to your numbers and still got SF = 1.51

Screenshot_20201128-203045.png
 

remingtonman_25_06

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I think you're getting that number because the bullet length isnt right in the library. It's only showing the bullet as being 1" when it is actually 1.053" I make sure to enter the bullet length manually after measuring with my calipers. I'm still getting a SF of 1.31 either way with a G1 or G7 formula.
 

Tripodmvr7

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SF increases as the bullet moves away from you. It might be low at the muzzle but stabilizes enough to ensure accuracy. Being a varmint bullet any tumbling of the projectile is to your benefit when it hits target.
 

BallisticsGuy

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That 1.5Sg for 'full stability' is terribly misleading and so some pedantry seems appropriate here.

A projectile is either stable or not. It's binary. There are not gradations of stability. Just like there are not gradations to being pregnant.

That said, there is a difference between how a projectile behaves when it's just stable enough to not keyhole versus where it's stable enough to keep the nose pointed straight forward after it's left the muzzle. If it's not spinning hard enough the tip can begin to precess or nutate, actions which by their nature expose more of the bullet to the oncoming air and thereby increase drag which we see as a reduction in B.C. when we try to match what we see downrange to what a calculator predicts. This is why guys like me that shoot 115gr DTAC's from 8 twist barrels don't have any more trouble than having to figure out how much BC it's costing us. My bullets are advertised at .620 G1 but I'm only getting about .57x-.58x G1 because I'm not spinning them hard enough which would require a ~7.5 twist.
 

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