Today, with an average 50-pound-draw compound bow generating 55-60 foot-pounds of kinetic energy (fpe) and an average hunting arrow zipping along at more than 260 fps, you might think complete penetration on big game is guaranteed. But I’m here to tell you it’s not.
A handful of years ago, I bowhunted caribou with two good friends. Unfortunately, my buddies experienced less than satisfying results in terms of arrow penetration.
Both shot their first bulls at 20 yards. The hits were great—dead-center in the lungs—but neither arrow exited the other side of those broadside animals. Both caribou ran several hundred yards before they collapsed, and we didn’t find enough blood on the ground to fill a thimble.
Another of my friend’s hits was even worse. That animal was quartering away at a severe angle. The broadhead hit a rib, deflected along the outside of the chest, then stopped in the lower neck. We recovered the bull, but it wasn’t pretty.
The sixth and last caribou of the trip was gratifying. My friend’s arrow hit broadside between the ribs, passed completely through, and buried itself in a stunted spruce tree beyond. The bull staggered 40 yards and pitched on his nose.
Why the difference in arrow performance on these animals? When we analyzed each episode back at camp, the reasons weren’t hard to figure out.
My friend who got shallow penetration on his first ’bou and a rib-skid on his second was shooting a powerful 70-pound-draw compound bow, but his arrows were fearfully light in weight. I generally shoot a total arrow weight (arrow and broadhead combined) of 8 or 9 grains per pound of bow draw. Using this formula, arrows from a 60-pound-draw bow should weigh 480-540 grains. But my fast-arrow buddy was shooting “5 grains per pound” arrows—only 350 grains from his 70-pound-draw bow. He was losing arrow energy because a lighter projectile always absorbs less power from your bow and peters out more quickly as it flies.
This guy’s second problem was broadhead choice. He was shooting the largest open-on-impact broadhead available at the time, and he was forced to shoot such a head because his arrows flew too fast and wobbled too much for stable flight with a fixed-blade broadhead. A bit more bow tuning or a heavier arrow might have helped, but this guy was trapped by his imperfect setup.
I’d tested his broadhead style the month before our hunt by shooting it through layers of vegetable-tanned cowhide. On impact, it lost approximately half its energy as it butterflied open. The result was half the penetrating depth of a streamlined cutting-nose head used with the very same bow and arrow shaft.
On my pal’s second caribou, one blade of his three-blade mechanical head had caught hide first on the sharply angling animal. The arrow had flipped sideways and deflected along the outside of the ribs. Only a large cut through the neck muscles had weakened and eventually dropped the bull.
My other friend’s problem was entirely different. A shoulder injury had forced him to use a 45-pound-draw bow with 80-percent let-off. This combo produced low kinetic energy. He was shooting a large broadhead with three fixed blades and a big nose cone up front. Blade and nose friction on impact were substantial, creating problems with any solid rib hit, but between the ribs this head performed extremely well.
Lessons Learned
On really big animals, you need four things for maximum arrow penetration. First, use the most powerful bow you can shoot well. In a compound bow, higher let-off might be easier to hold back but yields less penetrating energy. In other words, 65 percent let-off is better than 80 percent if you want maximum penetration, and 50 percent is better than 65.
Second, shoot moderately heavy arrows. Multiply your bow’s peak draw weight by 7 or 8, and you’ve got a practical arrow weight in grains. For example, a 60-pound-draw bow should shoot an arrow weighing at least 420-480 grains (broadhead weight included). The heavier the arrow, the deeper the penetration. For every 50 grains of extra arrow weight you shoot, you’ll pick up an additional 1 or 2 percent of point-blank arrow energy and up to 5 percent more energy at 40 yards.
Third, be sure your bow is tuned for perfect arrow flight. If an arrow wobbles through the air, it won’t fly with maximum accuracy, and it won’t absorb as much energy from your bow. A wobbling arrow also sheds energy as it flies, and as it penetrates big game.
Finally, choose a streamlined, low-friction broadhead for really large game. Cutting-nose, fixed-blade heads are generally best, but heads with small chisel points are also deadly. There are a few mechanical broadheads with low-friction entry systems, but most squander a lot of power on impact.
All else being equal, two-blade heads penetrate better than three-blade heads, and three-blade heads penetrate better than four-blade heads. Two blades tend to leave less blood on the ground, so I favor three blades for most of my hunting.
What about kinetic energy? For 350-pound caribou, 50 fpe should be a bare minimum. For 800-pound elk and muskox, I favor at least 55-60 fpe. For moose, polar bears, brown bears, bison and other North American species weighing 1,000-2,000 pounds, I believe 60-65 fpe is essential. If you use high-friction broadheads, you’ll need to raise these kinetic energy recommendations by at least 20 percent.
You can easily calculate your setup’s kinetic energy by first shooting an arrow through a chronograph at your archery store, then plugging arrow velocity and arrow weight into the following formula: fpe equals arrow weight (grains) times velocity (fps) times velocity (fps) divided by 450, 240.
The best way to instantly increase penetrating power is by simply cranking up the draw weight of your bow. Obviously, you must re-tune the bow after you do this. With an average compound bow, increasing draw weight 5 pounds will increase arrow energy about 10 percent. In magnum-sized game, this extra power might make all the difference.