This post provides a primer on recoil, and then explains how I measured recoil, including the approach and specific equipment used. It also reviews the rifles and cartridges tested.
What do we mean by recoil?
I’ve read a thick stack of articles and white papers on this subject, and suffice to say we can get pretty deep in the weeds if we weren’t careful. But, I’m going to try to keep it simple and on point. For our purpose, recoil is the rearward kick we feel when we fire a rifle. I’m less interested in the esoteric mathematics-based models (although they can be helpful) … my primary goal is to quantify the shooter-felt recoil as best I can. Specifically I want to measure the force felt at the shoulder of the person squeezing the trigger.
Let me lay a quick foundation of the basic principles, which we’ll build on throughout this post. Recoil is a reaction force, according to Newton’s 3rd Law: “For every action there is an equal and opposite reaction.” That means for every force there is a reaction force that is equal in size, but opposite in direction. Or stated another way, in a closed system the total momentum is constant. Momentum is simply mass × velocity. Here is a diagram that illustrates this for our scenario:
The recoil force felt by the shooter has 2 components:
- Acceleration of the bullet
- Acceleration of the gases created by the combustion of the gun powder
#1 is easy to calculate, but #2 can’t be accurately calculated (especially when using various muzzle brakes or suppressors). Since the force felt by the shooter is a combination of those two forces, to get the full story on recoil, it must be measured instead of calculated. Professor Hall, a Mechanical Engineer at the University of Texas, explains:
“A standard historical method used to quantify gun recoil is to calculate the ‘recoil energy’ that a shooter absorbs when firing a gun. This calculated recoil energy usually takes into account the primary recoil only [#1 above] … While this method of quantifying recoil is useful, it ignores several factors that can affect the actual recoil force (felt recoil) transmitted to the shooter. Consequently, it would be very useful to be able to quantify the actual recoil forces experienced by the shooter to assess how they may be affected by such factors as secondary recoil [#2 above], recoil reducing devices, stock dimensions, bore geometry, and barrel porting.” (Hall, 2008)
I couldn’t agree more, professor.
How does a muzzle brake reduce recoil?
To reduce the reaction force coming backwards (i.e. recoil), we have to address what caused it. Of the two components that cause recoil (mentioned above), nothing can be done to reduce #1. If we want to launch a certain size bullet at a specific velocity, there will be equivalent momentum in the opposite direction. We may be able to spread that force out over time with a recoil pad or other technique, but it is coming … we can’t do anything to reduce #1.
However, we can do something to reduce #2, which is the recoil related to the expanding gases. When you fire a rifle with a bare muzzle, all of the gases are expanding out of the barrel away from the shooter … which causes an equal and opposite force back towards the shooter. But, if we divert a portion of the expanding gases perpendicular to the bullet path, #2 is reduced. The diagram below shows this in action. The red arrows result in recoil. When part of the gas is diverted perpendicular to the barrel (blue arrows), those forces won’t contribute to recoil.
We can even go a step further by harnessing the energy of that gas to do work in our favor. When gas strikes the baffles of a brake and changes its direction, the gas is essentially pushing the rifle away from the shooter and reducing recoil even further. If we angle some of that gas rearward, it amplifies that effect even more. But, in the section on muzzle devices from Dr. Carlucci’s textbook, Ballistics: Theory and Design of Guns and Ammunition, he reminds us “Best design practice is to divert gases to the sides of the weapon, because rearward diversion could affect an exposed gun crew.” During my tests, a manufacturer sent me a prototype of a muzzle brake with 45° baffles back toward the shooter. It did offer amazing recoil reduction, but while testing that brake, a friend helping me with the tests caught some shrapnel in his side. It penetrated 2 shirts and caused a wound deep enough to see flesh. So there is clearly a downside to rearward deflection, but now we understand the effect that type of design can have on recoil.
What Contributes To Perceived Recoil?
Ultimately, I want to know what the difference in recoil is from the shooter’s perspective. I’m not interested in textbook theories, but the end-result of what I feel when firing a rifle. But, when we move to the subject of perceived recoil, we suddenly find ourselves in a strange mix of physics and psychology.
If we are firing a 140gr bullet at 2700 fps or 3200 fps, we can expect more recoil from the higher velocity. But according to Dr. Birch, an expert on this subject at the Impact Research Centre of Liverpool University’s Department of Engineering, “even when the velocity of two entirely different cartridges is similar, shooters may perceive a difference in kick. The explanation is usually found in the signature, which would have the same mean value but a completely different set of peak forces. Peaks in the signature contribute significantly to the difference in ‘feel’ between cartridges.”
Dr. Birch explains the force we perceive as recoil “varies considerably during a shot and is not just a simple push into the shoulder but a complex series of pulses occurring over a fraction of a second. Unfortunately, our nervous system is not sensitive enough to distinguish between these individual force pulses, which we perceive to be a single kick.” Here is an example of a recoil signature from my test that illustrates his point:
The recoil a shooter feels can vary by many factors, including:
- Physical build and stature of the shooter
- Shooting position, how tightly you grip the gun, how the rifle is supported, and how your body tenses in anticipation of recoil
- Stock design & fit
- Butt pad and clothing
- How many rounds you’ve fired that day (recoil can have a cumulative effect)
- Environmental stressors
Many of those factors are related to the connection between the weapon and the shooter. Felt recoil is largely dependent on the stiffness of that connection. If you were shirtless and firing a rifle with a steel buttplate that is pressed straight into your collar bone … that stiff connection is going to provide the full, unadulterated experience of the rifle’s recoil.
Would you be cold in a room that is 65° F (18° C)? Would you be hot at 80°? Some might be, and some might not be. Likewise, a shooter’s ability to tolerate recoil is based on personal perception. It was once widely believed that a 30-06 was the largest cartridge a man could shoot without developing a flinch. Many might argue that point today. (Read a good article by David Petzal on this.) This isn’t a test of manliness, but simply what you’re personally comfortable with.
The human factor simply isn’t calculable, so there doesn’t seem to be a way to accurately measure perceived recoil directly. But, that doesn’t mean we can’t still do something to gain insight into this area. Since we can’t directly account for those other factors, we can take an “all things being equal” approach for those elements, and instead try to quantify the percent difference in recoil reduction muzzle brakes provide. So even if a shooter had a rifle that didn’t fit them well, and they had a bad shooting position, and they’d already fired 100 rounds that day, and the rifle didn’t have a butt pad (ouch), if we measured a 20% decrease in recoil at the rear of the rifle with Brake A and a 50% decrease with Brake B … we could say with some confidence they’d experience less recoil with Brake B. Now, we could not say whether it would hurt, or even have an absolute value to quantify what they’d feel. But we’d know that with all other factors being equal, Muzzle Brake B would reduce the recoil felt at the shooter’s shoulder.
Does total force or peak force matter more?
When we say “reduce recoil,” what exactly are we talking about? That gets us to a big question:
I’ve asked a lot of sharp guys this question, and there seems to be proponents on every side. It is definitely not a straight-forward question, and there doesn’t seem to be an established industry standard.
Total “Size” of Force (i.e. Impulse or Momentum)
If you said total “size” of the force matters most, you’re in good company. Many take that stance. When I say the total “size” of the force, I’m referring to the area below the curve when you graph force over time (like the diagram above). In physics, this is referred to as impulse, which is simply the change in momentum. That is what you’d be quantifying if you used a classic ballistic pendulum, or a more pragmatic alternative I’ve seen is mounting a rifle on the sled, firing it, and measuring how far it moves. But, a rifle could move to the same point slowly or quickly, and while the size of the impulse may be identical … the perceived recoil could be very different.
This is often evident when people describe the felt recoil of a particular gun/cartridge as “soft” or “sharp.” Soft recoil is spread over a longer amount of time, where sharp recoil occurs over a shorter amount of time. Pistols are a great example: Some guys prefer the recoil of a 45 ACP over a 40 S&W. The 45 produces more recoil (i.e. impulse), but the recoil isn’t as sharp as a 40. They describe the felt recoil of the 45 as “more of a push,” and the 40 as “sharp and snappy.”
The diagram below shows two recoil forces that have the same total “size” of force (area under the curve), but would feel very different to the shooter. So we’re saying the blue area is roughly the same size as the red area. While it may be true that the cumulative “size” of the force is the same for both of those shots, the perceived recoil of those two recoil signatures would be very different.
Think about it like this: I have a bunch of beach sand in a pail. I pack it, flip the pail over, and make a little sand castle, like #1 shown below. But then, I push that sand castle over, and now it looks like #2. In both scenarios, it’s the exact same amount of sand. I didn’t add sand or take any away. They look quite different, but all that changed is the shape. The force curves in the diagrams above are similar to the sand castle … one is just pushed over, but its same “total amount of force” or impulse.
A recoil pad actually has this effect, as does a gas operated semi-auto. Both spread the recoil out over time, but they don’t change the total amount of force. They just push the sand castle over. They “allow the momentum exchange between the gun and shooter to occur over a longer period of time, reducing peak recoil forces,” explains Professor Hall.
So while total “size” of the force (otherwise known as impulse or change in momentum) certainly affects perceived recoil … it doesn’t seem to give you the full picture, because if we spread that same amount of force out over time we may perceive that as less recoil.
Many believe the peak force is what most closely correlates to perceived recoil. In fact, a team of mechanical engineers at Cal Poly clearly stated: “The amount of ‘kick’ or push against the shooter is determined by the peak force.” They’re saying the highest point of the line on the graph is most indicative of whether someone would say the recoil of a gun was more or less than another gun.
Dr. Birch seems to agree: “It is likely that a significant contribution to any discomfort and injury is due to the peak force in the early stages of the reaction. This catches our nervous system off guard well before our reflex has time to operate and can set up serious shock waves in the arms, shoulder and back; high-speed cine recordings confirm this.”
But, peak force alone doesn’t seem to tell the full story either. Two rifles could have the same exact peak force, but feel very different to the shooter. In the example below, if recoil from the blue curve hurt, then the red one might make a grown man cry … yet both have the same peak force.
So if you used a force gauge that only measured the peak force, that wouldn’t give you the full story either. It seems to be some combination of the two, so you can’t just measure one or the other. To ensure we’re making a valid comparison, we’ll need to capture the full recoil signature.
The Test Equipment & Setup
I looked at a lot different equipment setups, from academic white papers to creative and pragmatic DIY approaches on YouTube. I can appreciate both. But, as we saw above, to get the full story, you really need to capture the full recoil signature, not just the total momentum or peak force. The systems shown to the right are a few examples of setups I uncovered that were able to do that.
The problem is the recoil event is very short, lasting only about 10 milliseconds. If you were only able to record a few readings in the recoil cycle, you could easily miss the highest point of the peak. I wanted equipment capable of recording at least 100 force measurements in that time frame. That recoil force can also be in excess of 1,000 pounds. That is a lot of force, in a very short amount of time … which translates to expensive equipment.
I met several interesting people with experience measuring recoil, mostly for government projects. One group measured the g-force acceleration experienced by a scope under recoil. They attached an accelerometer to the rail of the rifle, which I considered. But, remember my primary goal is to quantify the shooter-felt recoil as best I can. Specifically I want to measure the force felt at the shoulder of the person squeezing the trigger. So I preferred to measure recoil at the butt of the rifle, which gets me closer to what I’m trying to quantify and it will include any effect from recoil pad, stock design, and other recoil reduction devices.
Unfortunately, there aren’t any pre-packaged solutions for this. After reading other research, I created the rough mockup below and sent it to a lot of smart engineers to verify I was headed in the right direction. Then I contacted several sensor companies to get recommendations and quotes for a system that would meet these requirements.
This design minimizes moving parts. As the team of engineers from Cal Poly explains, “less moving mass … will lead to more accurate force results.” Professor Hall also reminds us “It is important to note how the degree of restraint provided to the firearm … affects gun movement and the measured recoil force.” So I didn’t want to restrain the firearm in any unnatural way. I used two shooting bag, similar to the Cal Poly team: “This final design, unlike the concept design, incorporates two shooting bags instead of having the rear of the stock on a linear precision rail. It was determined that the friction force from the rear shoot bag is negligible…” This setup also closely mimics a precision shooter using a rear bag, which is the scenario I’m trying to recreate.
Sensors & Electronics
It was surprising how low-level I had to get to create this system. I ended up neck-deep in technical specs, doing all kinds of calculations, and even discussing custom parts and cables. It’s no wonder most manufacturers don’t have equipment like this … it was a lot of work to design and implement!
I learned most force gauges can’t come close to capturing data at the speed I was looking for. In fact, a few people claimed recoil happened so quickly that you couldn’t accurately measure it. But, I’m stubborn … so I pressed on. It turns out you can use a piezoelectric sensor, which generates a voltage when compressed. These sensors have an extremely high natural frequency.
Quotes for setups were all over the map, ranging up to $5,000+. I ended up talking to a rep at PCB Piezotronics, who had experience building a similar setup to measure recoil for a U.S. military project. The system we ended up building was also similar to what a team of mechanical engineers from California Polytechnic State University designed for Weatherby in 2013. They posted a detailed project summary, which you can view here. They explained, “In order to remain competitive, Weatherby needs an accurate method for measuring the recoil force of their rifles, shotguns, and ammunition. Currently no commercial product has been located for testing their guns, so no methods are being employed; however, it appears that other firearm manufactures have this data, so custom systems exist.”
Here is the set of equipment I ended up using:
I have a piezoelectric force sensor that can measure up to 5,000 lbs. of compression force in 1 lb. increments. The sensor is connected to a signal conditioner (amplifies signal and preps it for digital conversion), and that connects to what is referred to as a DAQ (Data Acquisition) unit. Some high-end DAQ devices have a signal conditioner built-in, but if you’re value-engineering, you can save money by getting separate devices. The result is the same. The DAQ device converts the analog signal from the force sensor to a digital signal and passes that to a computer through a USB cable. The DAQ device I selected is able to capture up to 48,000 samples per second. On the computer, I used DAQami software to capture the force readings and save them to a file. The post-processing and analysis was done by custom software I personally wrote.
The force sensor was professionally calibrated by PCB Piezotronics before my tests to ensure it accurately measures force data.
Custom Test Fixture
I designed and custom-made a test fixture that will hold the rifle and sensor. It was a heavy, solid design using 2” square tubing made from of thick, 11 gauge steel. I fully welded all of the joints. Honestly, its over-the-top … but I didn’t want any possibility of deflection skewing the results.
I tapped the back plate of the test fixture so the force sensor could be screwed directly into it (following the manufacturer’s recommendations). A mounting stud was included with the PCB sensor with 10-32 UNF-2A threads.
Part of the over-the-top strength of the steel base is for weight. As the Cal Poly team explains: “Since there is minimal upwards force, the weight of the system should be enough force to keep the system from lifting upwards … Rifles are designed to shoot accurately, which means that the rifles primary motion is in the direction of the barrel axis. This system is designed to record the backwards recoil force along this axis using the force sensor, meaning that the back plate of the system is going to be experiencing most of the force from the discharge; hence, the safety of the back plate is of utmost importance. The front of the rifle stock may “kick” upwards; however, this force is not substantial enough to be of any concern, as it will not overcome the weight of the system.”
If we just put the butt of the rifle directly against the force sensor, the recoil pad will deform under recoil and allow the force sensor to dig in at a single point. This could skew the readings or make them inconsistent. That type of setup also wouldn’t be recreating a shooter with the recoil pad on their shoulder. Once again, I was inspired by the design of the Cal Poly team, and here is what they said on the subject:
“The butt of the gun is placed against a piece of HDPE plastic that is secured in place with Velcro. This is to ensure that that the sensor is able to utilize the damping of the entire butt pad and measure the force that the shooter will actually feel. The plastic piece also provides a flat surface to contact the force sensor, which is located directly behind the mold and is attached to the back plate of the system. The flat surface will make sure that the force sensor does not encounter a bending moment, which could harm the sensor.”
Like Cal Poly, I used HDPE (High-Density Polyethylene), which is a plastic with a very high impact strength and great durability. I custom-made this from a 1″ thick block of HDPE I bought off Amazon. I used 1″ webbing straps with ladder-lock buckles to attach the holster to the stock. This setup was very flexible and worked well on every stock I tried it on.
The total cost of this recoil test system was around $1,500 (including hardware, software, tax, shipping, etc.).
It was found to be sensitive enough to measure recoil force differences among different factory loaded ammunition, and differences in recoil force of different firearms shooting the same ammunition.
Here is a what the whole setup looked like:
Here is a quick video of me running through a series of measurements with this test equipment with a specific rifle and muzzle brake.[youtube http://www.youtube.com/watch?v=j8JhsTT_JRs]
I tested on a range of rifles and cartridges in 6mm, 6.5mm, and 30 caliber, which represent the most popular calibers for precision rifles (based on muzzle brake sales). Let’s start by looking at the cartridges used in the tests:
The 6XC, 6.5 Creedmoor, and 308 Win are all mid-sized cartridges with a very similar powder capacity. They only vary by caliber size. I used popular bullet weights in each of the cartridges, which in the precision rifle world are typically the heavier bullets for each caliber. These cartridges will help us see how bullet weight and caliber-specific muzzle brakes affect performance.
I also tested with a 300 Norma Magnum (not to be confused with the old 308 Norma Mag). This is a newer cartridge that has been gaining popularity since Berger’s release of 230gr Hybrid bullet a couple years ago. The ballistics of this cartridge can challenge the 338 Lapua Mag at long-range (see Todd Hodnett’s comparison). The 300 Norma Mag cartridge has 11% more powder capacity than the 300 Win Mag, and represents one of the largest 30 caliber magnums. So it will help us see what happens to muzzle brake performance when you significantly increase powder charge and bullet weight.
I used match ammo for all of the cartridges, because I wanted to minimize shot to shot variance of muzzle velocity. The recoil measurement equipment is extremely sensitive, and I didn’t want factory ammo with a 20+ fps standard deviation in muzzle velocity to skew the results. I wanted the most consistent data possible, which meant using match-grade ammunition for all of the tests.
I strategically selected the rifles I used for the recoil tests, because I wanted to represent a wide range of stock designs and rifle weights. Here are the rifles I tested with:
The 6XC is one of my personal rifles built by Surgeon Rifles, and I covered that rifle build in-depth in a post last year (view that post). It is in a Manner’s 100% carbon fiber TF2 Tactical Folder stock. It has a thick 24” MTU barrel, and was the heaviest rifle used.
The 6.5 Creedmoor was a Surgeon Rifle one of my good friends let me borrow for the tests. (Thanks Rick!) It is in the very popular McMillan A5 tactical stock. It has a fluted 22″ Heavy Palma barrel, and the weight represents a typical precision rifle. Many rifles used in the Precision Rifle Series have weights similar to this rifle.
The 308 Win rifle I used is not necessarily a precision rifle, but it represents something most people would be familiar with. It is a standard, budget-friendly 308 rifle. This particular one is a Savage Axis SR 308 Win. I consider it my baseline for comparison. While not all of my readers have had the pleasure to shoot a really high-end precision rifle (like the other 3 rifles shown), almost everyone has fired a rifle similar to this. So by using it in the tests, it can help give some context for the amount of recoil we’re talking about. This rifle will also help us understand how the rifle weight plays into the recoil equation, and how that affects muzzle brake performance as a percentage of recoil reduction.
Lastly, the big 300 Norma Mag rifle was another rifle one of my good friends let me borrow for the tests. (Thanks Bob!) This rifle is in the popular Accuracy International AX chassis, and has a 27” Lilja Palma 30B contour barrel (similar to a Medium Palma contour). It’s also a heavier rifle, but represents what is typical for these types of large magnums. They can be unpleasant to shoot if you drop the weight.
References & Further Reading
If you’re interested in digging deeper into the stuff covered in this post, here are a few of the best resources I found in my research. Most are available for free online.
- Birch, Dr. Robert. “Recoil – Frequenty Asked Questions.” Clay Shooting Magazine (2001).
- Canfield-Hershkowitz, Benjamin, Trevor Foster and William Meijer. Rifle and Shotgun Recoil Test System. Poly Tech Mechanical Engineering Senior Project. California Polytechnic State University, 2013.
- Carlucci, Donald and Sidney Jacobson. Ballistics: Theory and Design of Guns & Ammunition. CRC Press, Taylor & Francis Group, 2008.
- Hall, Matthew J. “Measuring felt recoil of sporting arms.” International Journal of Impact Engineering (2008).
- Hokin, Samuel. “Gun Recoil.” BSharp.org: The Physics of Everyday Stuff.
- Lee, Joon-Ho, et al. “Experimental Performance Analysis on Recoil Pad for Reducing Firing Shock Force.” NDIA International Infantry & Joint Services Small Arms Systems Symposium. 2008.
- Nailon, Larry. “A Look At Recoil.” Clay Shooting USA (2006).
- Richmond, Michael. “Force, Momentum, and Impulse.” Rochester Institute of Technology Physics Class Lecture Notes.
- The Physics Classroom. “Momentum & Impulse Connection.” The Physics Classroom.
- US Army Materiel Command. “Engineering Design Handbook – Gun Series – Muzzle Devices.” 1968.
- Zhang, Huanhao, et al. “Investigations on the exterior flow field and the efficiency of the muzzle brake.” Journal of Mechanical Science and Technology (2013).
Other Post in this Series
This is just one of a whole series of posts related to this muzzle brake field test. Here are links to the others:
- Field Test Overview & Line-Up: Overview of how the tests, what brakes were included, and which were caliber-specific.
- Recoil Reduction Results: Let’s get right to the meat!
- Recoil Primer, Test Equipment & Rifles: Explains how I tested, and what equipment and rifles were used.
- Results for 6XC and 6.5 Creedmoor: Recoil results for the mid-sized 6mm and 6.5mm rifles.
- Results for 308 Win and 300 Norma Mag: Recoil results for the mid-sized 30 caliber and large magnum 300 rifles.
- Summary: Overview of recoil results from all rifles, and overall ratings of each muzzle brake.
- Ability to Stay on Target: Lasers and high-speed cameras were used to objectively quantify how well each muzzle brake helps you stay on target through a shot.
- Sound Test: A high-end sound meter was used to measure how loud each brake was to the side of the rifle and at the shooter’s position behind the rifle.
- Muzzle Blast & Ground Signature: High-speed videos were shot of each brake to show the direction of the muzzle blast, and the impact that could have on the shooter.
- Overall Summary: Putting all the results together in a summary that is easy to take in, and do side-by-side comparison, allowing you to draw your own conclusions on what muzzle brake is best for your situation.