Terminal bullet performance, the penetration and deformation of a bullet inside an animal or testing medium, is controlled by impact velocity. A bullet that behaves in a certain manner at a certain velocity can behave radically different at higher or lower velocity. Drive any expanding bullet fast enough, and it will come apart during penetration. Slow the same bullet down enough, and it will not expand at all. This makes it very important to know how your bullet will behave at the velocity it will impact animals because we rely on expansion and penetration to put meat in our freezer and trophies on our walls.
 

Impacting at 1811 feet per second the Swift Scirocco II bullet is just beginning to expand. This limited expansion allowed the bullet to penetrate the deepest of all tested at any velocity.
At nearly 2,500 feet per second the Scirocco II is beginning to upset into the classic mushroom shape hunters expect. This is also the impact velocity where the wound cavity starts to really develop.

    A hunter can take the word of bullet manufacturers as to where a bullet’s upper and lower velocity performance levels are,or he can find out on his own. I would strongly recommend the latter, especially if you are pushing bullets more than 3,000 feet per second or are shooting at extremely long ranges. High-velocity impacts at close range can rip some bullets apart and the range to the target will dictate the impact velocity. The way a bullet behaves at 50 yards may have little resemblance to how it behaves at 450 yards.
    At the 2006 SHOT Show I visited with Bill Hober of Swift Bullets. I was telling him how much I liked the .264 Winchester Magnum, and he gave me a box of his new, 130-grain, 6.5 mm Swift Scirocco II bullets. Hober was confident this new second-generation bullet would be just the ticket for the .264 and told me they were a true improvement over the original Scirocco. The original Scirocco was a good bullet that retained weight well but it upset too wide, limiting penetration too much for my liking.
    Testing bullets at low velocities from a .264 Winchester Magnum is not an easy proposition, so a 6.5x55 mm was used for the velocities less than 2,700 feet per second. Since the 6.5x55 mm uses the same diameter bullet as the .264 this worked perfectly. In all, the Scirocco II bullet was tested at eight different velocities simulating impact velocities from about 10 yards out to 600 yards, which is much further than I intend to shoot any big-game animal. Charlie Sisk of Sisk Rifles loaned me a custom .264 Winchester Magnum and a 6.5x55 mm to launch these bullets.
    For a bullet testing medium I used the Bullet Test Tube. Actually, I re-cast the expansion material used in the Bullet Test Tube. I have a large supply of this material on hand leftover from the initial consulting work I did for Ballistic Technology, the manufacturer of the Bullet Test Tube. I just heated the material up and poured it into new molds. Some of the material was of different colors because during initial product testing various trial colors were experimented with before the neon green color was selected. With the exception of the volumetric measurements of the wound cavities, which can only be done in the Bullet Test Tube, any bullet expansion medium that offers consistent results could have been used for penetration measurement and bullet recovery.

Bullet Expansion/Penetration Test Results

    Total penetration depths ranged from a low of 12 1/2 inches to a high of 22-plus inches. I was unable to list an exact penetration at 1,811 feet per second because the bullet completely exited the end of the Bullet Test Tube Xtender, punched through the metal cap and was found lying on the testing table.

A close look reveals the black polymer tip that was shed from the bullet near the entrance point. The cavity shows how the bullet began to yaw just after  it lost the tip but straightened out. Impact velocity was 2,102 feet per second.

    Surprisingly, the shallowest penetration occurred at intermediate velocities. There are several reasons for this. Weight retention was similar throughout the tested velocities, varying only 10 percent from the lowest to the highest impact velocity. At lower velocity expansion was minimal and did not drastically inhibit penetration. Finally, when expansion became fully initiated at about 2,200 feet per second, the frontal diameter of the bullet remained reasonably consistent, varying no more than .05 inch as impact velocities rose to up 3,190 feet per second. Since frontal diameter and weight were reasonably consistent between 2,289 and 3,190 feet per second, as velocity increased so did penetration.
    An interesting phenomenon is how the volume of the wound cavity grew larger as velocity increased, but the retained weight and expanded frontal diameter stayed relatively unchanged. This is another attribute of high velocity. If a bullet can take it, it will impart more tissue destruction as impact velocity increases. My tests with other bullets number into the hundreds, and another trend I have noticed is how a wound cavity tend to be larger as the bullet sheds more weight during penetration. The Scirocco II does not loose much weight but, you can see the how the weight it did loose did not affect the potential for tissue damage.

With an impact velocity of 3,066 feet per second in the bullet test Tube it’s easy to see that expansion occurs almost immediately. Comparing the wound cavity to one impacting at less velocity makes it clear the bullet speed is the determining factor of a bullet’s performance in game.

    Shedding weight is not necessarily a bad thing and can be a good thing. However, too much of a good thing can be bad. A bullet that sheds too much weight can deliver shallow penetration. The wound cavity may still be large, but if it is not deep enough it may not destroy the vital tissues necessary to deliver a quick and clean kill.
    It takes a test like this to illustrate how this juggling or balancing act plays out and is one of the reasons that I refer to bullets that behave like the Scirocco II as a “balanced bullet:” one that effectively balances expansion and penetration, offering the best attributes of both.
    Swift’s Scirocco II bullet is a very balanced bullet; well suited for big-game animals out to 475 yards when fired from a .264 Winchester Magnum and about 350 yards when fired from a 6.5x55 mm or .260 Remington. At those ranges the bullet will impact at about 2,200 feet per second; will reliably expand and give adequate penetration for most big-game animals. This is also the impact velocity where the wound cavity starts to take shape, and you should expect adequate tissue destruction on up through the higher impact velocities.
    In attempts to find an accurate bullet to hunt with hunters let money slip from their hands each year like it’s running out of a powder measure. Often in this search they never concern themselves with terminal bullet performance. Accuracy is important but, the bullet must work when it gets there. I did not conduct accuracy testing or do any load development with the 6.5 mm Scirocco II. However, the maximum load tested used 68.5 grains of Ramshot Magnum powder produced a three-shot, 1 1/4-inch group at 100 yards—not bad for a first try. The prudent hunter will evaluate his bullets at potential impact velocities before he goes afield. This will instill confidence and identify the maximum effective range—upper and lower velocity limits—of his rifle and bullet combination.