The question of what arrow weight someone should shoot for any given big game animal is a tough one to answer. The discussion is widely debated and there are a seemingly endless number of factors to consider. The discussion has been largely centered on penetration, or potential penetration, but before I dive into that topic it’s important to note that, personally, I place more weight in accuracy over potential penetration. For instance, if I had two arrows built and one only offered me 12 inches of penetration but hit the exact center of the vitals every time and the other arrow gave me a complete pass but impacted vitals a lower portion of the time, then I’m choosing the first option. Now, by saying that, it doesn’t mean that they aren’t mutually exclusive — it’s not that you can’t have an accurate arrow that doesn’t also penetrate well. You can; however, within reason, I place more emphasis on accuracy over penetration. Mainly, I want an arrow that tunes well, flies and groups well and has a flat enough trajectory that if the yardage is not exact that it still offers me a good chance of hitting the vitals — and I want enough penetration potential to kill the animal, obviously. So, let's look at kinetic energy, momentum and velocity,  particularly how it relates to vertical impact points at a variety of ranges. 

Kinetic energy

Regardless of where you land on which is more important — kinetic energy or momentum — kinetic energy is the most common metric to determine how well an arrow may penetrate. If you were to research how much an arrow should weigh to hunt any given game animal, the results are likely to be shown in KE — aka kinetic energy. Kinetic energy is a concept of physics. It’s a way to quantify the energy that an object possesses while in motion. Kinetic energy is equal to arrow weight, multiplied by arrow velocity (squared), divided by 450,800. The calculation can be done with your calculator on your phone, but you can also find many websites that will allow you to input your arrow weight in grains and your feet per second (FPS) and provide an estimated kinetic energy. In order to calculate your kinetic energy, you will need to know how many FPS your bow is shooting the arrow, which can be done by using a chronograph like the Garmin Xero C1 Pro Chronograph.

Here are some examples of arrows I recently built to test out:

498 grain arrow traveling 285 FPS and a 420 grain arrow traveling 310 FPS.

498 x 285 (squared) / 450,800 = 89.7 foot pounds

498 x 81,225 / 450,800 = 89.7 foot pounds

420 x 310 (squared) / 450,800 = 89.5 foot pounds

420 x 9,6100 / 450,800 = 89.5 feet per pound

Below are the common recommended kinetic energy recommendations for hunting.

Kinetic energy calculators will give you an estimate as the arrow leaves the bow, but there are also programs like Precision Cut Archery that will offer you the kinetic energy measurement at various yardages. For example, the 498 grain arrow traveling 285 FPS yields 89 foot pounds as it leaves the bow, 82 foot pounds at 30 yards and 76 foot pounds at 60 yards. Tools like this are great for helping you understand what the kinetic energy for your setup will look like and what your effective distance per kinetic energy might be depending on your specific setup.  

Momentum 

Momentum is what many suggest is the better metric to potential penetration. The difference in kinetic energy and momentum can be confusing — even to me as I write this article. To somewhat simplify it, kinetic energy is the energy an arrow possesses while in motion while momentum is the ability to maintain energy upon meeting resistance. As I understand it, using the same arrow builds I referenced earlier, a 498 grain arrow traveling 285 FPS has a momentum of .63 slugs. A 420 grain arrow traveling 310 FPS has a momentum of .58 slugs. The heavier, slower arrow carries more momentum, meaning it's harder to slow down when it meets resistance. A slug is a unit of mass that, when accelerated by a net force of one pound, results in a rate of acceleration of one foot per second squared. Momentum is calculated by multiplying the arrow's weight (in grains) by its velocity (FPS) and then dividing by 225,218. 

Below are the recommended slug units for bowhunting. 

Velocity (FPS) 

While velocity of the arrow is a portion of the discussion for both kinetic energy and momentum, the reason I wanted to highlight velocity specifically is to explore it as it relates to accuracy and point of impact over distance. If you have bowhunted long enough, you may have encountered a situation where a bull elk is approaching your calls or a mule deer that’s gotten up from his bed and moved a distance before offering you a shot. In both scenarios, you have probably used your rangefinder to range several objects (trees, rocks, bushes) prior to those situations unfolding to give you an idea of how far the range might be. I’ve done that so that I don’t have added movement/noise and so that I don’t have to shift my focus from drawing and executing a shot when it presents itself. When you do that, it’s not uncommon to misjudge the animal by a few yards. We could debate whether or not someone should shoot at an animal without the exact yardage, but the reality is that it’s a rarer occasion to actually be able to range the animal and shoot without getting busted due to the movement and time it takes to do so. I talk to many people who misjudge elk and deer, thinking that they are closer than they appear because their body size is so much bigger than a 3D or paper target. Ideally, you would have the exact yardage; however, practically, bowhunters are regularly estimating the shot and the flatter the trajectory of the arrow, the more likely they might be able to hit vitals if they guess the animal is closer than thought. So let’s look at the arrow drop of a variety of arrow weights. These numbers are taken while using Precision Cut Archery’s online program, configured for a Mathews Lift X 33 at a 30 inch draw length and 70 pound draw weight.

Next let’s consider the kinetic energy, momentum and the point of impact between these two arrows.

Discussion

In my situation, both arrows are above and beyond the recommended kinetic energy feet per pound and slugs for big game in North America, including elk, mule deer and antelope. The drop in inches between the arrows is interesting to think about given the other two metrics are within the recommended parameters. If I compare the two arrows, the heavier one will drop about 8 inches more than the lighter option at 60 yards. In a hunting situation, it could be the difference. I’m not arguing that the 420 grain arrow is the arrow I should choose to hunt elk, but it’s interesting to consider. 

At the beginning of the article, I stated that I think you should choose the arrow that tunes well, flies straight, groups the best — and meets the metrics for kinetic energy and momentum as well as your personal preference for arrow drop. Beyond that, there are several factors to penetration, including the broadhead you choose, how true the arrow impacts and the animal itself (how it's orientated, its reaction, whether or not you impact bone versus soft tissue). 

By and large, when setting up a bow and arrow weight combination, you are starting with a set draw length and draw weight. Your draw length is the length of the power stroke behind the arrow; the longer the draw, the more power will be transferred. Similarly, with draw weight, the higher that number, the more force that will be put into the arrow. Your draw length is, for the most part, non-negotiable. You should shoot the draw length that offers you repeatability and stability in your aim so it will land where it lands. Your draw weight should be set at a level where you can comfortably draw the bow from every shooting position and during any weather condition. If you can do that and practice with it often, have no irritation or injury from it, then increasing your draw weight for hunting is beneficial. For most male bowhunters, the draw weight is going to land between 55 to 70 pounds and for most females it will be between 45 to 60 pounds. Everyone is different. If your draw length and draw weight are set in stone, then the way I see it is that you can trade momentum for velocity or velocity for momentum. You could increase the arrow weight, which will increase momentum and decrease velocity. You could reduce arrow weight, which will increase velocity and decrease momentum. Both velocity and momentum are important, but I might suggest that if you have more than you need of one that you trade some for the other. 

Examples for the average bowhunter and lower draw weight and draw length bowhunters are in the table below. Keep in mind that these numbers are not set in stone. They’re estimations and may be slightly higher than what you might encounter in real life. Every bow and individual will have specs that are uniquely their own; however, this is generally an idea of what you could expect and is built to highlight potential numbers that you can use to make the best choice. Again, I would reiterate that my own personal recommendation is that the tune of the arrow, accuracy and grouping is the highest priority (within reason). Also, consider the animal that you are pursuing. For medium-sized game animals like deer or antelope, increased FPS might offer an advantage while penetration is somewhat less of a concern. For larger game animals like elk or moose, I may lean slightly more towards momentum and penetration than higher FPS (within reason).

In the end, what everyone really wants to know is the answer to this question: What grain arrow should I shoot? As you see from the article, my hope is that readers will geek out, consider their own objectives, crunch the math and come up with an arrow that is best suited for them. However, if I had to, here are my general recommendations. I’d suggest a moderate approach. For elk, a 440 to 500 grain arrow traveling at a rate of 275 to 300 FPS with a front of center of 10% to 15%. The 275 to 300 FPS range is moderate enough that a mechanical broadhead will tune well as will most fixed blade broadheads. At the higher FPS range of that recommendation, a fixed blade head might require some micro tuning of the bow.  For deer and antelope, I would suggest a similar arrow construction. If I were to change anything, it might be that I would increase the FPS by decreasing the total arrow weight some. For example, a 410 to 460 grain arrow traveling at 285 to 315 FPS with a 10% to 15% front of center. When you approach those higher speeds, tuning the broadhead tipped arrows is critical (not that it isn’t always), but with faster arrows it seems to be even more important.

A note on broadhead choice: I’ve killed animals with fixed blades and I’ve killed animals with mechanicals. Generally, all things considered, I find mechanicals to be more forgiving and tuneable while cut-on-contact fixed blades have better penetration potential. Overall, shoot the broadhead that you are confident in. Given the efficiency of modern compound bows, almost anyone can shoot either a fixed blade or mechanical and have enough energy and momentum to obtain pass-through shots. The only suggestion I might offer beyond that is that if you are low on energy/momentum because of your draw weight and length, a fixed blade can improve the penetration potential since it requires less energy to push through hide and tissue. 

Hopefully, this helps some with your arrow selection. Good luck and shoot straight this fall!