lnewqban,

Dimples aren't helpful in every situation. The drag of a sphere is strongly affected by the location of the flow separation. Dimples force the boundary layer to be turbulent, which moves the separation location rearward, which reduces drag. For some projectiles, the drag may not be strongly dependent on the separation location, or their shape may result in a fixed separation location. In cases like this, forcing a turbulent boundary layer doesn't help, and probably increases drag. For example, a classic bullet shape, with a squared-off rear end, will experience flow separation at the square corners, whether the boundary layer is laminar or turbulent, so dimples can't shift the separation location, and wouldn't reduce drag. Some wings do have the equivalent of dimples, used to tailor which part of the wing experiences separation first, or to prevent a laminar separation bubble. The drag on a submarine is significantly higher for turbulent flow, so dimples would not be used to reduce drag on a submarine.

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