Energy transfer – some things you may not know
by B.B. Pelletier
Today’s topic was suggested by dsw. I’m going to explain energy transfer from a practical standpoint, then I will try to back up what I say with a few facts.
The question I’m answering is, “How does a pellet transfer energy to its target?” If that is different from what dsw asked, I apologize.
1. Energy is transfered as deformation and heat
When a moving lead pellet is stopped suddenly by a hard immovable surface, such as the steel back of a bullet trap, it deforms on the surface of the steel. Up to about 550-600 f.p.s., deformation is all you can see, but if you were to grab the pellet an instant after it stopped, it would be very hot, too. This heat is also part of the energy the moving pellet gave up when it suddenly stopped. If the pellet hits the steel faster than 550 f.p.s., small pieces of it break off and fly away, taking energy with them. At 700 f.p.s., some of the pieces are so small that we call them dust, and at 800 f.p.s., the heat from impact is so great that some of these pieces of dust are instantly melted. To the shooter, they appear to be sparks in the bullet trap.
2. Wound channels
Animals are softer than steel, so the pellet tends to punch through the tissue, destroying it as it goes. Here is what DOES NOT happen: A pellet has virtually zero hydrostatic shock, which is how modern small-caliber bullets kill game. To get that kind of tissue-destroying shock requires much more velocity than pellets can achieve. The wound channel of the fastest pellet rifle made (which I believe is the AirForce Condor in .177 caliber) WILL NOT be significantly larger than the diameter of the pellet.
3. Energy is transferred as impact
Impact energy is what moves the animal. It’s the same energy that less-than-lethal weapons, such as shot-bag launchers, use to knock down people without killing them. A pellet that remains inside an animal transfers all of its energy to the animal. If the pellet goes through the animal and out the other side, only a portion of the energy is transferred. If penetration is not deep, the impact energy will be greater (proportionally) than if the penetration is deep and more energy is used to cut through tissue.
Impact energy demonstrated
The buffalo hunters used large-caliber (.40 to .50) rifles and very heavy bullets (350-550 grains). The best buffalo guns would not only kill with one shot at 300-600 yards, they also often knocked down an 1,800-2,000 lb. animal! In contrast, when a .30 caliber (.30/06) high-velocity bullet is used on buffalo, it can take 2 to 10 shots to put the animal down. The .30/06 develops about 2,800 foot-pounds of muzzle energy, and a .45-120-550 Sharps (the rifle Matthew Quigley shot) develops about 2,400 foot-pounds, yet the larger, slower bullet kills faster and moves the animal (knocks it down) when it strikes. This is an ideal example of the optimum use of energy transfer.
I didn’t make up those facts about the .30/06 and buffalo. That was from a Denver Post story in the early 1950s, when the city thinned a local herd of buffs to feed the poor at Christmas. Only one animal out of eight was killed with a single shot.
4. Metallic Silhouettes
The problem of energy transfer came up early in the sport of metallic silhouettes. In this sport, you shoot at steel silhouettes of game animals at various distances. The ram is the largest, heaviest target and is placed at 500 yards. All the silhouettes have a flat foot that rests on a stand. To get credit for a hit, the silhouette must be knocked completely off the stand. Shooters have tried accurate calibers such as the .243. They find that, even though the rifle has the necessary energy at 500 yards, it often strikes the target wrong and only twists it sideways. A larger caliber will completely spin it off the stand. The energy the larger bullet is able to transfer to the silhouette is greater than the energy the smaller bullet can transfer.
Energy transfer depends on what the pellet hits, how fast it’s moving when it hits and what the pellet is made of. A pure lead pellet holds together well. A hard alloy pellet tends to break up earlier, and a synthetic pellet often separates in game. Once it does separate, however, the metallic core stays together.
What the pellet hits is extremely important. A freight train moving 10 mph has many millions of foot-pounds of energy. Yet it can hit a rabbit jumping across the tracks and not bruise him. But if it hits an automobile – look out!
5. How to improve energy transfer
Slow down the pellet! That usually means shooting heavier pellets, although some airguns also have power adjustments. Use an inefficient pellet nose shape. Wadcutters are very inefficient penetrators, as are hollowpoints, like the RWS Super-H-Point.
Energy transfer is important to the airgun hunter or pest control shooter. We work with such a small energy budget that we cannot afford to waste anything. Learn what it takes to get the most from your airgun.