Patched Round Ball: Weight, BC & Twist
Estimate a lead round ball's weight, rough ballistic coefficient, and a traditional twist-rate guideline.
How this works
A lead sphere's weight comes directly from its volume and lead's known density, so knowing just the diameter is enough to calculate it precisely. Ballistic coefficient for a sphere is rougher — spheres don't have a stable orientation in flight the way a pointed bullet does, so the form factor (roughly 1.8-2.2 relative to the G1 reference) varies more between individual balls than it does for conventional bullets.
Where 1:48 twist comes from
The Greenhill twist-rate formula, when a projectile's length equals its diameter (true for any sphere), simplifies to a fixed 1:48-inch twist recommendation — which is exactly why that twist rate became the traditional standard for round-ball muzzleloaders long before ballistic calculators existed.
Frequently asked questions
Why is a round ball's BC so much lower than a modern bullet's?
A sphere is a very inefficient aerodynamic shape compared to a pointed, boat-tailed bullet, so even though it may weigh a similar amount, it sheds velocity far faster — which shows up as a much lower ballistic coefficient.
Why does a round ball vary more in BC than a regular bullet?
A sphere has no fixed orientation in flight the way a pointed bullet does, so its effective form factor can vary more between individual balls — this calculator's BC estimate should be treated as rougher than a typical modern-bullet BC.
Is 1:48 twist right for every round ball caliber?
It's the traditional rule of thumb, derived from the Greenhill formula reducing to that fixed ratio whenever a projectile's length equals its diameter (true for any sphere) — a solid starting point, though some rifles are built with other twist rates that also work well.
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