Do Small-Diameter Shafts Maximize Arrow Penetration?
August 25, 2015
Our ongoing Practical Bowhunting Test Series attempts to answer age-old archery questions, test commonly held (but unproven) theories, assess claims, and tackle myths. In this test, we take a look at the widely held belief that using small-diameter arrow shafts results in deeper target penetration.
The idea that a small-diameter shaft will penetrate deeper than a larger-diameter shaft makes sense, since the smaller diameter results in less surface area and, therefore, less energy-robbing friction. But is a small-diameter arrow really all it takes to boost penetration?
What if the narrower shaft is much lighter than the larger-diameter arrow? Do I need to somehow match the mass weight of the two arrows to evaluate the true benefit of the smaller shaft? And does it matter what the outside surface material is? For example, will a carbon micro-diameter arrow penetrate as deeply as an aluminum-cased arrow of the same size and weight? We set out to answer these questions and more with our testing.
Before diving into our tests and results, let's discuss the question "Does this really matter that much, considering modern bowhunting equipment and a high number of pass-through shots?" Well, I would argue that it does indeed matter.
We all have different abilities, different equipment setups and a wide variety of animals we pursue. Some bows are slower, some animals are thicker and not all shots are perfect! On any given shot in the field, penetration may become VERY important, and for that reason every bowhunter should be interested in maximizing the effectiveness of our equipment.
We started with the new Mathews NO CAM HTR bow, which was set to a 29-inch draw length and 60-pound draw weight. An NAP QuikTune 3000 arrow rest and nocking loop were added to the bow, and it was then tuned for good arrow flight out of our exclusive Petersen's BOWHUNTING X-Ring Shooting Machine. The range was set up for a 20-yard shot from the front of the bow to the face of the target.
Next up was choosing the target material, and since we've had excellent results in the past with our shop-made foam plate targets, we decided to stick with them. We cut 16x16-inch squares, stacked them horizontally and wrapped them with industrial-grade plastic stretch wrap, which holds the target together.
Some readers will surely ask (or at least wonder) why we used this material, as opposed to commercially produced targets or even a lineup of deer carcasses. My short answers are consistency and practicality. The material we use is readily available, affordable and we can cut and wrap a target in less than 10 minutes.
More importantly, it is a very consistent medium that performs well shot after shot. Consistency is what counts in comparative testing and we certainly had that, with three-shot groups never varying in depth of penetration by more than half an inch with fieldpoints or fixed-blade broadheads.
Although conducting a similar test with deer carcasses would surely be entertaining, inconsistencies from one spot on a carcass to the next — not to mention from one carcass to the next — simply would not yield the kind of consistency we require.
All the test arrows — six different models with the same approximate spine — were cut to 28.75 inches and fitted with fieldpoints that were either exactly the same (where possible) or at least the same basic shape.
For the purpose of our test, we categorized our test arrows into three general diameter categories — small (Easton Carbon Deep Six Injexion and Deep Six FMJ), medium (standard Easton FMJ and Easton Axis) and large (Easton XX75 aluminum and Beman ICS Hunter Patriot Edition).
Test 1: Matched Arrow Weight
In this test, we found the heaviest arrow in the bunch (465 grains) and brought all the others up to the same reading on the scale. Why did we do this? Simple; it singles out diameter as the only variable in the test. In this way, we can know whether diameter alone is having an impact on the depth of penetration.
Three shots were taken for each arrow with a fieldpoint and then again with a 4-blade fixed broadhead. A brand new broadhead was screwed on the end of the arrow for each shot. Broadheads were not used twice. The results in the charts below represent the averaged penetration for all three shots.
To give you an idea regarding how well matching the arrow weights leveled the playing field, it is worth noting that arrow speeds for all shots in the matched weight tests registered a speed of 251 or 252 feet per second, with kinetic energy for all shots only varying from 65.07 to 65.59 foot-pounds.
We originally decided to test just one 4-blade broadhead model and the fieldpoint. However, those results made me curious. So, we took a few more shots with a 3-blade fixed broadhead and a 2-blade mechanical head to see where things went. I had enough heads to do an entire round with the mechanicals but only enough 3-blade heads to test two arrows.
With fieldpoints, it is easy to see the distinction between both diameter and material, except that in the largest diameter the carbon penetrated deeper than the aluminum. After that, things get somewhat muddled.
Within the broadhead group, the 3-blade broadhead shows the clearest distinction in penetration between the two diameters, but the 4-blade head appears to have lost the pattern among the small- and medium-diameter groups, both of which still demonstrated more penetration than the largest diameter shafts.
There is a pattern in both diameter and material with the 2-blade expandable head, though the differences are slight, especially in the small and medium diameter sets.
Test 2: Varied Arrow Weight
In the real world, if a bowhunter decides to switch from their current arrow to something with a smaller diameter, he or she will most likely just go to the local pro shop and order the correctly spined arrows cut to length without making any modifications to mass weight.
With that scenario in mind, we performed a portion of the test again with no changes to the factory mass weight of the various arrows. We simply cut them to the same length and shot them as is.
As you would expect, arrow speeds obviously varied in relation to mass weight. Despite that, kinetic energy for all arrows in this portion of the test remained very close, with the total range only varying between 64.39 and 65.4 foot-pounds.
We conducted the same penetration tests for fieldpoints and the 4-blade broadhead as we did for the matched-weight arrows. Again, three shots were taken for each test model with a fieldpoint and then with a broadhead. A brand new, never used broadhead was screwed on the end of the arrow for each shot. Broadheads were not used twice. The results in the charts below represent the averaged penetration for all three shots.
Again, we can see a clear distinction of both arrow diameter and material with the fieldpoint test, other than the Patriot penetrating deeper than the XX75. The 4-blade broadhead results were much the same as those with the matched-weight arrows. There was little distinction between the small and medium diameter arrows. However, they all penetrated deeper than the largest diameter shafts.
Interpreting the Results
So, what does all this mean? Clearly, our results show that small-diameter shafts offer a penetration advantage over larger-diameter shafts — though there is no denying the level of advantage seems to vary based on what is screwed onto the end of the arrow (fieldpoint, fixed-blade broadhead, mechanical, etc.).
We found the variations in penetration among the 4-blade, 3-blade and 2-blade broadheads particularly interesting, and the switch from fieldpoints to broadheads seemed to reduce penetration more on the smaller shafts than it did on the larger shafts.
In fact, if you follow just the largest two test shafts through the tests, you will see there aren't any really significant differences regardless of head type, and the 2-blade expandable broadhead actually penetrated deeper than the fieldpoint!
Despite that apparent anomaly, we feel confident in saying that smaller-diameter shafts still offer a penetration advantage — not always a great advantage, but an advantage nonetheless. My broadheads change often depending on what and where I am hunting, and this test shows me I have the best chance at maximum penetration with a smaller-diameter shaft as I move from one head to another.
The test also shows a narrow but consistent advantage to aluminum shafts over carbon. I am giving small-diameter shafts the close win for this series of tests, with a best supporting cast award going to aluminum.