As I understand it, I think the theory behind both the Woodleigh Hydro solid and the NF CPS are similar. The same holds true, to an extent, for any non-expanding flat point solid. The small hollow point at the nose of the Hydro and the CPS creates a slightly larger high pressure gas cavitation envelope around the nose and front of the bullet. A little more with these hollow point designs and a little less with the plain flat point designs.
As the bullet flies through tissue it is stabilized by spinning, the tissue is just a much denser media than air. There is less spin-reducing-friction in the cavitation bubble formed, especially in wet tissue, so the bullet continues to maintain spin rate longer and is stabilized for a greater straight line distance through the tissue. Additionally, because both the FP and CP type noses create a slightly larger high pressure bubble around the nose and front part of the bullet, a slightly larger channel is created than would be the case with a non-expanding round nose or spire point. So, all in all, the results are a bullet that flies through the media/tissue in a straighter line which in turn increases depth of penetration, plus creating a slightly larger channel than would be the case for a round nose or spire point. Additionally, as the bullet slows during penetration, the center of gravity remains near center to slightly forward in the bullet thus aiding stability- as opposed to a non-deforming monolithic spire point which has by design a center of gravity slightly to the rear of the bullet... a somewhat unstable form as friction reduces the spin rate during penetration.
One on left is TSX from buffalo. Two on right are NF CPS from buffalo. An unfired bullet is included with each type for comparison. 416 Rem- impact vel approx 2250 fps.
