07-25-2011, 03:01 AM
JWH Wrote:Welcome, Colorado!
Regarding the "sweet spot," first, any caliber can have long, streamlined, high SD and high BC bullets. My opinion is that 6.5mm became a sweet spot because larger caliber bullets with the same SD and BC start to be heavier and generate more recoil than is desired. Yet smaller caliber bullets with the same SD and BC were traditionally less desirable for big game, four- or two-legged, and were unnecessarily light. Just a hunch. . .
John
Yes, geometry dictates that. As the radius of the projectile goes up, while keeping the same basic shape, it's volume goes up by the same amount roughly cubed. So the amount of lead goes up tremendously as the caliber goes up. Since the surface area only goes up to the square of the radius, you get less drag per grain as the bullet scales up.
So, if the bullet gets too small the drag goes up per unit weight, if the bullet gets too big the weight gets to be too much for a man-portable rifle to handle.
SD is more like a measure of puncturing ability. It takes a lot more force to punch a stick through a piece of cloth than a needle. If you put the same force behind the needle and stick, the needle will punch through easier. So high SD bullets (skinny per unit weight) punch through air and targets easier, other factors being equal.
There's a limit to SD, too though. To get heavier a bullet of a given caliber has to get longer, or heavier in other ways. That's why anti-tank rounds are made with depleted uranium, it's heavier than lead.
So all those factors converge around 6.5mm for a rifle with medium recoil.
Rick