Bullet penetration is a penetrating topic for most hunters. Some say you can have too much, some say too little.
    
What they're arguing is terminal performance, and penetration is just one part. Expansion and energy transfer are two more. Expansion increases surface area and tissue destruction. Energy transfer is more nebulous. Some shooters think a bullet must stay inside an animal. If it punches through, deadly energy is wasted.  
    
Bowhunters seek just the opposite. They love the pass-through arrow, knowing it creates the most tissue destruction and a blood trail. Rifle hunters don't necessarily “get” this, assuming they absolutely must have massive magnum energy. (Read more on magnum madness right here.)
    
Bullets sometimes kill instantly by “shock.” One deer punched through both lungs crumples in a heap. But the next deer, shot in the same place with the same bullet, stands as if not even touched. The next might race 50 yards before expiring. Why?
    
Some speculate that if the heart is in the process of pumping out a load of blood when the bullet hits, the massive energy within the slug is channeled through the already pressurized circulatory system to deliver massive shock.
    
Sounds like a stretch to me, given the slight pressure of a heartbeat compared to the thousands of foot pounds of energy in a bullet. You can see how shock energy from a .308 Win. cartridge loaded with a 130-grain Barnes TTSX bullet passes through a 6x6x16-inch block of ballistic gelatin in this video. How much can a heartbeat change that?
    
However shock happens, you cannot depend on it. The safest shot is heart-lung because it destroys the organs that pump blood to the brain. Blood pressure drops. The deer collapses. But it could run 10-400 yards first, thus the need for an exit hole for blood trailing.

Pictured here is a .17 Mach 2 bullet hitting a quart of water. Shock power.
Imagine what a .30/06 impact would look like. A heartbeat is going to change this?

Here's the lowdown on how some of the most popular bullets react when hitting a big game animal:

1. Full-metal-jacket (solid) bullets penetrate too much, zipping through with minimal tissue damage.
2. Frangible varmint bullets break up quickly. Sometimes this destroys heart/lungs for a quick, clean kill, but sometimes it merely ruins a bunch of meat without reaching the vitals.
3. Soft points (traditional cup-and-core bullets with soft lead cores in thin metal jackets) can mushroom perfectly, break into two or three pieces, or even flatten like a pancake, depending on where they land.
4. Bonded-core bullets usually expand less but retain more mass for deeper penetration.
5. Controlled-expansion bullets—either via internal walls, bonded cores, monolithic cores or combinations—expand 1.5-2X, retain 90 percent or more mass and pass through, even after striking major bones and muscle groups.  
   
What bullets should you use? The ones you believe in. Just understand their limitations and don't expect any to drop game in its tracks every time. 

What say you? Do you have a favorite bullet type? Share your comments below.