So reading a post here yesterday about poor accuracy at close range turning into better accuracy at longer ranges got me thinking about something that I read in one of Bryan Litz's books that's puzzling to me. There must be information that is missing or I'm just not smart enough to comprehend the physics behind it so here goes.
Any one can plug in numbers to a ballistic app and come up with an stability factor for the particular bullet they want to shoot. It's widely known that an sg of 1.4 should be adhered to for proper stability across a broad range of environmental factors. In reading the books, Brian has stated that a bullet that leaves the muzzle stable will always be stable and that sg can actually grow to 4-5 as it travels down range because it sheds velocity at a faster rate than the aerodynamic forces acting on it can force it to tumble.
This is where I'm getting lost. In looking at the actual formulas for calculating twist rates, all the variables of input are constant as the bullet travels down range with the exception of velocity which is always decreasing so the rpms of the bullet should also decrease at an equal rate, should they not?
Launching a bullet at 2800 fps out a 9 twist barrel equates to:
2800x720)9=224,000 rpm's with roughly an sg of 1.4 based on my environmental conditions. If I change the mv to 1120 fps we have this:
1120x720)9=89,600 rpm's with roughly an sg of 1.03 which is much lower.
So what part of this equation that makes sg grow down range am I missing?
Any one can plug in numbers to a ballistic app and come up with an stability factor for the particular bullet they want to shoot. It's widely known that an sg of 1.4 should be adhered to for proper stability across a broad range of environmental factors. In reading the books, Brian has stated that a bullet that leaves the muzzle stable will always be stable and that sg can actually grow to 4-5 as it travels down range because it sheds velocity at a faster rate than the aerodynamic forces acting on it can force it to tumble.
This is where I'm getting lost. In looking at the actual formulas for calculating twist rates, all the variables of input are constant as the bullet travels down range with the exception of velocity which is always decreasing so the rpms of the bullet should also decrease at an equal rate, should they not?
Launching a bullet at 2800 fps out a 9 twist barrel equates to:
2800x720)9=224,000 rpm's with roughly an sg of 1.4 based on my environmental conditions. If I change the mv to 1120 fps we have this:
1120x720)9=89,600 rpm's with roughly an sg of 1.03 which is much lower.
So what part of this equation that makes sg grow down range am I missing?
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