Tight groups

Why You Should Use Medium Weight Arrows

Shooting a bow from tree
In bowhunting situations where shots tend to be short and trajectory is not much of a factor, using medium to medium-heavy shafts that increase penetration makes all the sense in the world.

“Moderation in all things.” So said the Roman dramatist Publius Terentius Afer (190-159 B.C.) in a phrase that has been attached to many things in modern life. It has become my philosophy when building hunting arrows.

In a world where flash is cash and instant gratification the norm, many novice archers believe they need to shoot the fastest arrow they can shoot. Why? They’re not exactly sure. They just know that if they’re not sending a shaft off at 300-plus feet per second something’s amiss. And so they choose the lightest shafts they can and call it good, without understanding anything about terminal performance.

Before penning this piece I went back and looked at something I had written almost two decades ago on the difference in terminal performance between light and heavy arrows. It was written back when we all shot heavy aluminum arrows that, together with a broadhead, weighed somewhere between a 30-inch 2315 aluminum arrow that weighed in at 545 grains and left a 80-lb. bow at roughly 220 fps and a “lightweight” 30-inch 2413 aluminum shaft weighing in at 446 grains and leaving the same bow at 250 fps (a scorcher at the time).

Kansas whitetail
Whitetails do not take a lot of kinetic energy when it comes to being able to shoot an arrow shaft right through them. Even the largest bucks can be taken with a relatively light draw weight bow and proper arrows. Robb took this 300-pound live weight buck in Kansas in 2014 with a shaft weighing 410 grains.

Contrast that with what most bowhunters shoot today–compound bows that give us a raw arrow speed of somewhere between 260-280 fps, with some blasting over the 300 fps mark. Our arrows have lightened up as well. As an example, in 2015 my 70 lb. Mathews ChillR sent a 28 1/2-inch 330 ICS Beman Hunter Pro carbon shaft tipped with 100-grain Rage broadhead that weighed a total of 402.4 grains off at 283 fps.

The point is, hunting compound bows and arrows have evolved mightily over time. Today we are shooting fast arrows with lots of kinetic energy designed to make hitting the target easier than ever before, as well as provide the downrange “pop” needed to cleanly kill deer and other big game animals.

Admittedly, my set-ups are certainly not the fastest around. I have friends who get the Willies if they are not shooting 300 fps-plus. These guys achieve this speed using much lighter arrows than I prefer to shoot. Here’s why I prefer the moderately-heavy arrows.

Kinetic energy (K.E.) is the most important part of the arrow penetration story, though certainly not all. To determine how much initial K.E. your own bow-and-arrow set-up has, you need know just two things–how much your arrow shaft weighs, including fletching and broadhead, in grains; and how fast the arrow leaves the bow at the shot. You then simply plug the numbers into the standard kinetic energy formula, which is mass * velocity2/454,240.

In the above example, my arrow left the bow with about 70.94 ft./lbs. of K.E. This is a lot of kinetic energy–way more than most experts believe is the minimum for hunting deer-sized game. That minimum number has been debated around archery circles for decades, but most of the people I respect have settled on a number somewhere around 40 ft./lbs. of K.E. at the shot.

Robb AZ elk
You need deep penetration when hunting large-bodied game, something lightweight arrows will not give you. Robb took this AZ elk at 27 steps with an arrow weighing a total of 410-grains.

However, you must remember that, because arrows decelerate rather quickly as they travel downrange, the amount of K.E. delivered on the target will be much less than initially generated. That’s why an arrow that blows right through a deer at 20 yards may only penetrate about half the shaft length at, say, 40 yards.

Those light (8 grains/inch of arrow shaft length or less) arrows favored by many 3D shooters, speed freak bowhunters, and many archers who shoot light draw weight bows, soak up less of a bow’s stored energy at the shot than heavier shafts do. Any leftover energy not transferred from the bowstring to the shaft at the shot is instead transferred to the bow itself, which results in vibration. Those light arrows also soak up less energy at the shot and actually decelerate faster than heavier shafts the farther downrange they travel, and shed energy much more rapidly than do heavier shafts. That means that after a certain point, they deliver much less K.E. to the target. And trust me on this–even a difference of just 5 percent in K.E. can mean a big difference in penetration on game–especially bigger animals like elk, bears, big-bodied deer and the like.

K.E. is not everything in the penetration game. If it were, bowhunters would be shooting arrow shafts that were light as a feather, since the lighter the arrow, the faster it will initially blast out of the bow, and therefore it would generate more kinetic energy since velocity is squared in the K.E. equation.

Kansas whitetail
Whitetails do not take a lot of kinetic energy when it comes to being able to shoot an arrow shaft right through them. Even the largest bucks can be taken with a relatively light draw weight bow and proper arrows. Robb took this 300-pound live weight buck in Kansas in 2014 with a shaft weighing 410 grains.

The other scientific principle involved is called momentum. Momentum is defined as “a property of a moving body that determines the length of time required to bring it to rest when under the action of a constant force.” Bowhunters don’t consider momentum as much as K.E. for several reasons, the most likely being the fact that as the speed of an object changes (and after the shot, an arrow begins to decelerate, meaning that for every foot of flight it is traveling at a different speed than the foot before) it is too complicated to play with in a simple discussion. What you do need to know is that, all things being equal, heavier, denser objects traveling at the same speed as lighter objects will have greater momentum. That means it is more difficult to stop them, which means they will penetrate more deeply. An easy analogy here is visualizing both a golf ball and an egg smashing into your own ribs at the same speed. Which will hurt more?

Taking it a step further, when an arrow meets solid resistance at the target (think ribs or shoulder blades), its direction will be impacted. Again, the laws of physics are in play. A heavier body in motion has a tougher time changing direction than a lighter body, which means the heavier shaft will have less of a tendency to veer off target than the lighter shaft.

Another penetration factor is drag. In this case, that refers to the amount of resistance the shaft meets as it penetrates an animal. Small-diameter arrows–and today, there are hunting shafts with diameters as small as 4mm–have less drag than larger-diameter shafts, and thus will penetrate deeper than fatter shafts, all other things being equal.

This is why the medium-weight hunting shaft makes the most sense. When choosing a hunting arrow, you want to blend the best of all worlds. You want to shoot the fastest arrow you can shoot to help flatten trajectory, yet you need to have enough K.E. and momentum when the shaft strikes the target to ensure deep penetration–preferably a complete pass-through.

Modern broadhead
Among other things, medium-weight arrows help you achieve enough kinetic energy and momentum to allow today’s popular mechanical broadheads to perform properly.

You need that reasonably-fast arrow speed even when using a laser rangefinder because game moves around a lot, walking in and out of shot windows in the brush. You may have taken a 30-yard rangefinder reading off a tree trunk, but when the critter walks a few steps in front or behind the tree and you don’t have a time to range him so you know exactly how far he is, the raw arrow speed offered by the medium-weight shaft makes a killing hit easier to achieve than a super-heavy arrow. At the same time, it will deliver all the penetration necessary for a humane kill.

Bob Robb cape buffalo
Even super-heavy arrows have to fly perfectly. Robb’s 816.2-grain Easton Dangerous Game shaft, shot from 19 yards, is about to hammer this South African Cape buffalo.

So, how heavy an arrow do you need to shoot? For the average bowhunter with a draw length somewhere between 27 and 29 inches and a compound bow draw weight of between 60 and 70 lbs., using an arrow shaft that weighs somewhere between 9 and 10 grains/inch, minimum, will get it done. With my own 28-inch draw length, I cut my shafts to 28 ½-inches and try and get a total arrow weight, including broadhead, fletching and nock, of somewhere between 390 and 410 grains. If I can send that puppy off at somewhere between 265 and 285 fps from a 70-lb. compound, I am very confident I can hammer any big game animal on the continent, even at longer ranges.

In bowhunting, such confidence is a very good thing to have.–Bob Robb

10 thoughts on “Why You Should Use Medium Weight Arrows”

  1. Poppycock!!!! Shameful that SCI would even publish such non-sense. Momenteum of an arrow is what matters not KE. Just to many problems with this article to list. Really sad that this was not reviewed before distribution to our members around the world!

  2. The formula for momentum is mass multiplied by velocity or p = (m)x(v) and the formula for kinetic energy is 1/2 Mass times velocity squared or KE = (1/2 m) x v2. Both utilize mass and velocity but in different ways and both are useful to the hunting archer; however, momentum is generally considered more useful for determining penetration potential. I agree with the author that the arrow and broadhead weights he recommends for most hunters using 60-70 lb bows will probably work best as they provide hunters with arrows which, so long as properly aimed, will penetrate the vitals of and accomplish a killing shot on all North American game animals and, at the same time, will allow the hunter to have reasonably tight pin spacing and a reasonably flat arrow trajectory. The problem with using really light arrows is having too light a spine and the problems that causes. Overall and except for his characterization of KE as the most important factor in penetration, the article is helpful in pointing out that the hunting archer is faced with having to make compromises between different weights of arrows and broadheads in order to achieve optimal performance when it matters most.

  3. I always find it interesting how fast some are to pass judgment on any matter of discussion. I have been an archer for over 35 years and have seen and participated in all of the fads that have come and gone. I didn’t have a lot of access to experts in the field so I relied on whomever was willing to share information right or wrong. Both momentum and KE need to be considered and the fact that information is being shared on a medium like this is great. The author did state that momentum is key. The more important issue here is knowledge and sharing it. As hunters we now have to consider a lot more than we used to because of this type of medium as everyone has access to it. Anti’s could look at this differently, archers are uninformed and use the wrong equipment which results in an inhumane act. Crazy I know but education is paramount the more articles like this the better. We live in a world of bigger, faster, and stronger is better. When knowledge practice and awareness are key.

  4. Kinetic Energy is non-directional and is simply a measurement of the efficiency of a bow that allows one to estimate its ability to launch an arrow – it has nothing to do with penetration – that is physics. Using the same bow, I can generate a similar kinetic energy calculation using a light or heavy arrow, but the Momentum calculations are exponentially greater with the heavier arrow. For hunting arrows where penetration is key for quick, humane kills and not just touching a dot on the target, a much better article would have discussed structural integrity, arrow flight, forward of center, broadhead mechanical advantage (not to be confused with a mechanical broadhead), shaft diameter to ferrule diameter ratio, arrow mass/weight for increased momentum, broadhead edge/finish, shaft profile, broadhead/arrow silhouette, tip design, arrow mass to get through bone to the vitals when a less than perfect shot is made.

      1. I dang sure hope he does as what he is talking about is right in the proverbial “12 ring”!

  5. Worry about anti’s? Don’t worry they disdain us no matter how you KILL your intended quarry. I could not care less how they decipher this discussion as the goal is to decimate the BEST scientific based data available. And that article is NOT it…….fact based physics does not change. KE does not carry the weight (pun intended) that momenteum does, and that’s a scientific fact.

  6. The Dr. Ashby data is knowledge on steroids! The 650 grain threshold has proven golden for me.

Leave a Reply