It all boils down to the bullet. No matter what cartridge you choose or how accurate your gun, the only thing that actually impacts your quarry and brings it down is the bullet.
Today's ammunition for whitetail-appropriate cartridges offer a wide and often confusing array of bullet options and types, all claimed by their manufacturers to be "ideal" whitetail choices. Fortunately, most of them actually are. But at the same time, these various bullets also differ greatly in their design, impact characteristics, expansion and weight retention, and an intelligent understanding of their differences is a great aid in making a choice that will provide the results you need no matter what kind of shot you may find yourself confronting.
Compared to larger game animals, whitetails are relatively easy to kill. Their muscle density, bone mass, and overall body weight and dimensions are in fact very similar to those of a human being. It is no accident that many of our traditionally most popular whitetail cartridges derived from military rounds like the 7mm Mauser, .308 Winchester, or .30-06, and their many offshoots. However, traditional military-type non-expanding full-metal jacket bullets are not appropriate for hunting because they cause little organ damage and tend to fully penetrate, leaving a very small wound channel. The key to proper whitetail bullet selection is to balance penetration and expansion in a whitetail-size target, no matter what the angle of your shot into the animal might be.
For many years, the deer hunter's choice between penetration or expansion was mutually exclusive. You could have one but not the other. A varmint-type bullet that would expand destructively inside a deer with a broadside shot might not reliably hold together if it had to crash through shoulder bone on a quartering angle. A bullet tough enough to penetrate heavy bone or muscle might not expand once it got inside. Today, thanks to impressive advances in bullet technology, we can have our cake and eat it, too. There are a variety of "bullet families" currently offered in premium ammunition from all major manufacturers specifically designed to accomplish both goals, each offering slightly different approaches and slightly different sets of advantages or disadvantages for particular hunting encounters.
Traditional jacketed/lead core bullets, such as Remington's classic Core-Lokt and Winchester's Power Point, were the first effective effort to solve the bullet performance dilemma and have been used successfully by millions of whitetail hunters for many, many years. By enclosing a soft lead core with exposed nose within a harder copper jacket of increasing thickness from nose to heel, these designs were tough enough to get deeply inside, yet allow mushrooming of the lead nose for a more destructive wound channel as penetration progressed. The problem was that when lighter, faster versions of these bullets encountered heavier bone, the core and jacket might separate and fragment, losing ability to reach into the vital areas.
Partition-type lead core bullets, originally developed by John Nosler, were a second significant step. The Nosler Partition design employed an internal cross member in the copper jacket, separating the lead core into a nose section and a heel section. Upon impact the nose would expand (sometimes entirely shedding the nose core), but the cross member would preserve the heel core, providing sufficient integrity and retained weight for continued penetration even after striking bone. Partition-type bullets could be relied upon to retain at least 65 percent of their weight no matter what the angle of impact. They became widely popular among hunters of all types of game and spawned a family of derivative offshoots from a variety of other makers, such as Swift Bullets' tough-jacketed "A-Frame" series.
Bonded-Core bullets next began to appear on the market as technical advances in manufacturing technology provided improved ability to eliminate core-jacket separation and increase retained weight up to 80 percent while preserving expansion. As a category, bonded bullets employ a variety of manufacturer-specific techniques to "bond" the copper jacket to the lead core, some very effective, some not as much.
Overall, bonded designs provide a significant performance advantage over conventional non-bonded lead-core bullets and are available in a wide variety of styles from exposed-lead nose to polymer tipped designs for improved long-range ballistic efficiency. There are probably more types of bonded bullets on the market today than any other. One notable example is ATK's Fusion ammunition line, wherein a copper outer layer is electrochemically "fused" to an inner lead core, literally molecule by molecule, making core-jacket separation impossible from the tip of the nose back.
The Barnes X-Bullet represented a true revolution in hunting bullet design, creating an entirely new category and performance standard. Constructed entirely of copper, the X-Bullet entirely erased any core-jacket separation problem, and achieved tremendous immediate expansion via a deep narrow cavity in the nose that erupts into four sharp-edged petals (the famous "X") upon impact. Weight retention averages 95-plus percent (Barnes claims "virtually 100 percent"), and even though the copper X-Bullets weigh less than same-size lead-core bullets, they deliver more impact at the end because they shed less of their weight.
Consider: if a top-performing 150-grain bonded .308 bullet loses only 20 percent of its weight during impact, it penetrates as a 120-grain bullet inside the vitals. A 130-grain .308 X-Bullet starts out faster, and on whitetails typically carries at least 95 percent of its weight all the way to the end. Which hits "heavier?"
The X-Bullet format has also been adopted by a variety of other manufacturers, most notably in the former Winchester Fail-Safe and the current polymer-tipped Winchester XP3 bullet designs, which combine a Barnes-type copper front end with a rear lead core for increased adaptability to varied bore dimensions. Barnes itself has added variations: the ballistically improved polymer-tipped TTSX, and the tipped MRX featuring a tungsten-based heel core for shorter-by-weight bullet dimensions and increased case capacity.
What's best? They're all good. For whitetails, I personally want a bullet that will upset with immediate shocking effect for those perfect broadside shots into the heart-lung area, yet also have maximum weight retention for deep penetration to the vitals through shoulder bone. If you can't figure out from that what my own choice is, you haven't been reading very carefully.