To George Hill:
> I have observed over the years that some calibers prefer certain bullet types. IE - I have yet to see a load for the .45 that is better than the 230 grain Hydra-Shok. The Hydras-Shoks in other weights and other calibers don't seem to have the same mojo.
> Is there some physics going on in these different calibers that make a bullet behave differently in different calibers?
Physics is, of course, always involved in the way bullets behave (regardless of caliber). But looking at terminal performance from a more fundamental standpoint, _bullet geometry_ is the ultimate determining factor. As Fernando pointed out, the cavity dimensions, core hardness and jacket thickness are all regulating factors. Regarding the difference in performance from caliber to caliber, however, core wall _thickness_, the degree of wall taper and wall “length” (axial dimension) are the prime controlling factors. Wall thickness is obviously linked to cavity size but not necessarily in a proportional way from caliber to caliber unless the bullet was purposely designed to be proportional by deliberately scaling up (or down). Example: A .45 caliber bullet could be designed with the same proportional ogive, same proportional meplat diameter, same proportional cavity I.D. (at opening), same proportional cavity depth and same proportional cavity I.D. (at terminus) as that of a particular 9mm round. This type of ‘proportionality’ is rarely ever used by manufacturers though because of the often great disparity in muzzle velocity between calibers. In other words, the dimensioning approach cited above, while proportional, would not necessarily be conducive of optimum performance. Manufacturers attempt to make their bullets as robust as possible so that they perform acceptably over the widest possible range of use. The interior/exterior ballistic limitations of a particular bullet and the scope of application are always taken into consideration. Often a bullet is called upon to defeat sheet steel AND open up after being fired through fabric.
The wall area surrounding the .45 Hydra-Shok cavity is thick -- proportionally thicker than the .40 or the 9mm. Plus, the .45’s mass is greater and its muzzle velocity is lower. This means that the cavity wall expands at a different rate than say, a 9mm Hydra-Shok bullet. In the case of the .45 Hydra-Shok, the above combination happens to provide us with an efficient projectile. Part of this efficiency is due to cavity design and part of it is because of the heavy bullet’s thicker wall area and greater momentum.
It's interesting to note that almost everyone places great stock in the final diameter of the recovered bullet. Unfortunately, the recovered diameter tells us little about the in-transit performance of a particular round. For instance: WHEN did maximum bullet expansion occur in its transit through the test medium? HOW LONG was the maximum diameter maintained before the ogive material curled back to produce a smaller diameter? HOW MUCH energy was transmitted at inch 2 as compared with inch 4 and inch 6? What was the shape of the ogive at maximum diameter? Was it somewhat rounded or was it flat? Did the bullet have to penetrate 2 feet to attain maximum expansion? These are all important questions for which almost no one has answers (that’s why I use high-speed photography as an analysis tool). In the case of the thick-walled Hydra-Shok, its maximum expanded diameter is maintained for a longer period of time than the thin-walled 9mm. The same holds true for several other brands of bullets out there.
As mentioned, momentum is partially responsible for any heavy bullet’s expansion (the greater momentum applies greater pressure (crushing force) to the nose of the bullet)). A substantial percentage of this pressure is directed into the Hydra-Shok cavity. This translates to greater expansion than would have been the case if the bullet had had less mass. While a high velocity lightweight bullet may transfer more energy to the target (and expand well via sheer velocity), the heavier, slower-moving bullet expands proportionally easier and maintains maximum expanded diameter longer. This undoubtedly accounts for the .45 Hydra-Shok’s impressive street record. I should point out though that any heavy h.p. bullet has a distinct advantage over a lighter h.p. bullet as momentum helps it expand proportionally easier.
> I know this sounds like a stupid question - but I am looking for a formula here on bullet selection.
Unfortunately, there are too many variables involved to reduce bullet performance to a single formula. If such a formula existed, I’d be out of a job.
Tom Burczynski
[This message has been edited by Tom Burczynski (edited August 29, 1999).]
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