Firing a rifle is essentially a chain of controlled chaos. The explosion of the primer ignites the powder causing it to burn and become a gas. This hot, expanding gas creates pressure which forces the bullet through the barrel. Thanks to Isaac Newton, we know that each action has an equal and opposite reaction and that energy and momentum is conserved in a system with momentum equaling mass times velocity. Because the bullet and gas are being pushed out of the barrel, there must also be a force pushing back on the rifle. This is recoil. Because of conservation laws, we can’t remove momentum to reduce recoil but we can redirect it. That’s where muzzle brakes come in.

A little more technical information and then I promise I’m done. Momentum has to be conserved but velocity is directional. This essentially means that a force can be applied to redirect the momentum without breaking the conservation laws. This is why a muzzle brake is nothing more than a gas redirection device. So let’s look at the momentum of the system. To simplify everything we have the mass and velocity of the bullet, as well as the mass and velocity of the gas being pushed away from the rifle. Since mass is also conserved, we’ll assume all the powder gets burned and the mass of the gas is the same as the initial mass of powder. There’s also a correction factor for the velocity of the gas. Since it is pushing the bullet, it has to be moving faster than the bullet. Both of those are pushing away from the rifle resulting in the rifle being pushed back at some velocity. You can walk through the formula on the SAAMI website. https://saami.org/wp-content/uploads/2018/07/Gun-Recoil-Formulae-2018-07-9-1.pdf

This is the total momentum available for the system and assumes all gas and the bullet are being pushed directly away from the rifle. Meaning the velocity of the bullet and gas are away from the rifle.

Since velocity is a direction, the way we can change the momentum is by changing the direction of the velocities. We don’t want anything to interfere with the bullet so that leaves us with the gasses. Now the muzzle brake. The muzzle brake is little more than a wall with a hole for the bullet to pass through. The gas hits the wall, gets stopped from going forward, and is forced to change direction through vents. The more gas that wall stops from going forward, the more effective it is. The velocity of the mass has now changed direction. Changing the direction of the momentum.

Now, rather than having a large momentum pushing back on the rifle, it has been broken up into horizontal components. The gas hits the wall, gets stopped from going forward and pushes on the brake, then changes directions. The force applied from the brake to change the gas direction actually pushes FORWARD on the rifle, reducing the felt recoil. The more mass gets diverted, the greater the effect. How much the velocity changes also has an effect. Turning the gas back towards the rifle is even more effective than just turning it out the side. Since changing the direction of velocity effects momentum, the greater the change the greater the result. This is why some brakes are swept back. There are a few different styles of muzzle brake typically used that we will look at in another article. But this is the main idea behind how they work and some of the science that is used in the theory and design behind them.