Red, drag models overlook the force of supercavitation which occurs when a high velocity object moves through a fluid medium. While there is enough velocity to sustain a gas bubble around the bullet, there is no drag exerted directly on the bullet by the surrounding fluid or tissue, because the soft tissue and fluid are violently displaced by the gas bubble and neither actually touches the bullet until the velocity decays to the extent that the gas bubble ceases to envelop it. The higher the velocity, the greater the size of the bubble, relative to the shape of the bullet. This would account for Brobee's observation that the bullet with the faster impact velocity created a larger wound diameter and here is where the diameter and shape of the bullet play a role in the wound diameter. Provided that a bullet's entire energy can be absorbed by the target without exiting, the actual wound volume would not change with the design of the bullet, only the shape and depth of the wound are impacted. Again, the effects of supercavitation only apply with the passage of the bullet through soft tissue, and the bullet comes into direct contact with any bone it must penetrate, so drag applies. Still, there must be great resistance to the bullet's forward motion or the velocity would not drop off in a fluid medium any faster than it would in air. Perhaps then it would be like attempting to push a balloon through the water; the density and incompressibility of the water resists the forward motion of the gas bubble, which in turn indirectly slows the bullet.
When a bullet mushrooms in the tissue of a big game animal, the rate of expansion is determined by the design and construction of the bullet. The TSX and the Rhino bullets are similar to the extent that both have pleated jackets that are designed to fold back along predetermined lines, and it is hydraulic energy that accomplishes the task rather than simply bumping into a target of greater density.
In fact, when fired into a dry medium, the Rhino does not expand at all, it simply bends and hooks around.
This contrasts with lead core bullets using a tapered jacket to control the rapidity of expansion, or the dual core type bullets, and display some degree of expansion regardless of the medium they impact. Having said that, the wound volume of any given bullet of a certain weight and impact velocity, for example any 180 gr bullet impacting at 2500 fps, will be the same, regardless of the caliber or construction of that bullet, lethality is however another matter.
Some years ago, I was seal hunting with my trusty M-17 .30/06. Unfortunately for the seal in question, I made several bad decisions starting with a poor bullet choice. The Speer 130 gr hollow-points were wonderfully accurate in that rifle, with groups averaging 5/8ths of an inch. Now my intention was to make a head shot, because if you shoot a seal in the body, more often as not it is able to kick down into it's hole and is lost. This predetermination for making a head shot is why I was overly concerned about accuracy; a seal's head is a pretty small target. I was laying in some pressure ice watching a seal that was facing directly away from me. The seal would raise his head to look around briefly, then drop it back down again, before I was able to shoot. Then for quite some time he would not lift his head above the line of his back. Finally, I was going to have to shoot or forget it because lying on the ice was cooling me off and it wouldn't be long before I would start to shiver. This is where I made my second mistake. I decided I would attempt a body shot, with the idea that the bullet would break the spine and punch through to the chest. It seemed reasonable to me that with a severed spine, the seal would be unable to get into its hole. I was in a good solid position, aimed carefully, and pressed the trigger. The rifle boomed, and the seal was gone down it's hole faster than you could believe. Well, that's that I thought, stood up and prepared to return to the snowmobile, when all of a sudden the seal jumped back up on the ice. So I returned to my jack-ass prone position and started pounding him, the 5th or 6th shot killed him, with a hit in the neck. As I shot he was pulling himself around on his front flippers making a large red circle on the ice. My pal and I made a pretty good attempt of performing an autopsy. The initial shot was farther back than I had anticipated, I should have held higher for the range. Where the bullet hit bone, the bone was disintegrated and a large fist sized wound flecked with bullet metal remained, but there was no penetration beyond the bone. We determined that after the first shot he realized he couldn't propel himself in the water and would drown if he stayed submerged, so he came back up. Had I had access to .308/130 gr TSXs, the results would have been better for the seal and for me, but this was before Randy Brooks had made X bullets a commercial reality. So for those who think that accuracy is the overriding concern when choosing a big game bullet, or that one bullet is the same as another, take heed.
