September 06, 2012
We've all heard the story of William Tell, and most remember the gist of the tale. As punishment for failing to bow in respect to the newly appointed Austrian Vogt, Albrecht Gessler, Tell was commanded to shoot an apple off his son's head. Fortunately, his shot was true. But when the story is retold, tellers often err on one minor detail. Tell's choice of weapon was a crossbow! It's understandable how that detail might be overlooked — and at least in this case is of minor importance. But sometimes, the details do matter.
I recently came upon a series of figures distributed by the North American Bowhunting Coalition comparing the trajectory and kinetic energy of modern crossbows and modern compound bows. At first glance, the differences appear rather significant.
The trajectory of crossbows and compound bows was similar out to about 25 yards, and then they diverged dramatically. The crossbow bolt was only about four inches low at 50 yards while the compound's arrow had dropped to 40 inches. At 70 yards, the crossbow bolt was 20 inches low, compared to 120 inches for the compound arrow. And at 100 yards, the crossbow bolt had dropped only 60 inches compared to 240 inches for the compound arrow.
The differences in kinetic energy were even more dramatic. The crossbow started at 190 foot-pounds, compared to 70 for the compound. At 100 yards, the crossbow bolt was still carrying 140 foot-pounds of energy, while the compound arrow was down to a mere 40 foot-pounds — an astounding difference.
Taken at face value, it certainly appears the crossbow has a clear advantage when it comes to trajectory and kinetic energy. When I went back and read the fine print, however, a slightly different picture began to emerge.
The "modern crossbow" used for this comparison was a PSE TAC 15 set at 170 pounds and shooting a 425-gain bolt at 425 feet per second. The "modern compound bow" was a Mathews Drenalin set at 60 pounds and shooting a 540-grain arrow at 241 fps.
Of particular interest was the testers' choice to use AMO (Archery Manufacturing Organization) standards for the compound, which calls for a maximum draw weight of 60 pounds and a 540-grain arrow (18 grains per inch) rather than the more commonly used IBO (International Bowhunting Organization) standard of 70 pounds of draw weight and a 350-grain arrow. Ten more pounds of draw weight and 190 fewer grains of arrow weight make a significant difference in both trajectory and energy. Under IBO specs, that same Drenalin will shoot a 350-grain arrow at 320 fps (a 75 percent increase), with almost 80 foot-pounds of energy.
The reason, without going into too much detail, is that kinetic energy equals mass times velocity squared (K=mv2). Because the value for speed is exponential, any changes in speed have a much greater effect on energy than do changes in weight. By going to a lighter arrow, you gain more speed and flatter trajectory with relatively less energy loss.
You can see this concept clearly illustrated each month in Bowhunting's High Grade Bow Reports. For example, if you turn to page 142 of the September 2012 issue, you will see that the 2012 Ross XD set at 29 inches and 65 pounds will shoot a 425-grain arrow 255 fps with 61.38 foot-pounds of kinetic energy. But going to a lighter, 375-grain arrow resulted in a speed of 268 fps, an increase of 5 percent, while kinetic energy decreased just 2.5 percent to 59.82 foot-pounds.
It's also interesting that PSE's TAC crossbow — which operates on an AR-style firearm platform — was chosen for this comparison. While the TAC could certainly be used for hunting, it stands in a class by itself even among modern crossbows.
A Broader Look
Digging a little deeper, I found another crossbow vs. compound bow analysis from several years ago that compared a handful of top models from both categories. The very fastest crossbow in that comparison shot a 425-grain bolt at 405 fps and generated 155 foot-pounds of energy, while the rest produced bolt speeds that ranged from 320-350 fps and kinetic energy that ranged from 95-132 foot-pounds. When compared to five of the fastest compounds, generating arrows speeds from 314-366 fps and kinetic energy from 77-97 foot-pounds, the ranges of the crossbows and compound bows actually overlapped.
But, like I said, that was several years ago, and as we all know, shooting technology keeps advancing. So, just to ensure I didn't miss anything, I also examined several more recent comparisons, as well as manufacturer's specifications collected for our annual crossbow roundup. Not surprisingly, the ranges and averages were similar and overlapping. Generally speaking, crossbows hold an edge over compound bows of roughly 30 fps of speed and 30 foot-pounds of kinetic energy. However, that disparity seems less important when you observe nearly the same difference exists among different models in each category. In other words, the fastest compound bow on the market has roughly the same speed and energy advantages over its slower compound competitors as a top-of-the-line crossbow has over a compound of equal quality.
In one of the comparisons I looked at, the fastest crossbow on the list had a trajectory advantage of just 12 inches at 50 yards over the fastest compound on the list. When you consider a deer's reaction time, the additional noise of a crossbow and the fact that most shots at deer (compound or crossbow) are taken at less than 25 yards, differences in long-range trajectory — anything over 50 yards — become less important.
Several points can be taken from these comparisons. In terms of efficiency, the crossbow does have an advantage, ranging from slight to significant, depending on whose figures you care to use. But since the overriding goal of any hunter should be to make a clean, efficient kill, it only makes sense to choose the most efficient weapon, whether crossbow or compound, and the trend in both categories is clearly in the direction of faster, lighter models that generate more than enough power to take down even the biggest of big game.
Perhaps the most important takeaway is that if you are going to compare crossbows to compound bows, you've got to be fair. Comparing the fastest, most technologically advanced crossbow on the market to an average compound bow further restricted by a subjective testing standard (AMO) is not comparing apples to apples — something legendary crossbowman William Tell is quite familiar with.