Editor’s Note: This article is an excerpt from Gunsmithing the AR-15: The Bench Manual by Patrick Sweeney.
Flash hiders and muzzle brakes (or compensators) are two types of muzzle devices, and they use gas created from firing a cartridge differently:
– Hot gases from the muzzle flash expand into the cool air, emitting photons from heat(not from burning powder). A flash hider hides most of this by creating turbulence. By mixing the cooler ambient air with the expanding bubble in a random pattern, there is no longer a large flat surface, or bubble, to glow.
– A muzzle brake jets the gases out to the sides or top, or causes them to slam into a fixed barrier, decelerating them. You can use the leverage the barrel provides, and not just negate the added gas component to recoil, but pretty much negate all the recoil. That’s what a muzzle brake does.A good muzzle brake is loud, as the gases are re-directed back at you or the guy next to you.
What does a flash hider do, and what exactly is flash?
Flash hiders keep muzzle flash down in the visible spectrum. In the infrared, they don’t do squat. In fact, a rifle may produce a visible muzzle flash only part of the time, but through a thermal scope it will flash every time. Hot gas is hot gas, even if it isn’t incandescent in the visible spectrum. A thermal scope will see it every single time.
Disregard the notion that flash is unburnt powder. If you have not combusted all the powder the case contained in its travel through 16 inches of bore, you’re using the wrong powder. In some handgun cartridges, the powder has been fully combusted before the bullet leaves the case. In rifles, the powder has all been burned by the time the bullet is 3 or 4 inches down the bore. Actually, the bright flash we see is incandescence.
The powder burns at a high pressure and temperature. The confined combustion temperature of your average rifle powder is on the order of 3,500 to 3,700 degrees, Fahrenheit. That gas leaves the muzzle in an expanding bubble. At that temperature, the gas is incandescent and the glow happens to be in a part of the spectrum we can see.
Also, the uncorking pressure of rifle ammo, typically 10,000 PSI or higher, produces a smooth-surfaced gas bubble. It expands too quickly to mix until it has sufficiently cooled.
An example of incandescence is your common light bulb. The electricity flows through the filament, and the filament glows due to the electrical resistance of the material involved. The filament, due to electrical resistance, is heated up until it is hot enough to be incandescent, and the glow is in the visible spectrum, which throws off … light. The filament is not consumed, it is not burning, it simply ejects photons due to the heat. Turn the light off, electrical flow stops, and the filament cools and stops emitting photons.
The muzzle flash of your rifle is the same principle. The hot gases, expanding into the cool air, emit photons from heat, not from burning powder. A flash hider hides most of this by creating turbulence. By mixing the cooler ambient air with the expanding bubble in a random pattern, there is no longer a large flat surface, or bubble, to glow.
The flash hider on your AR-15
An AR barrel with no flash hider is problematic. In a rifle-length 20-inch barrel it isn’t too bad, as there is enough length and time for cooling and pressure loss to keep flash manageable. Also, there are a lot of uses where the shooter doesn’t really care if there is a visible flash or not. A deer hunter, for instance, cares not one whit about muzzle flash. Target shooters are not bothered by it. Most matches are held in broad daylight, and cardboard targets aren’t going to notice muzzle flash. But the rest of us want a flash hider.
The basic A1 and A2 flash hiders do a very good job. The A2 differs only in that it has three of the slots closed, so that when you are firing prone, in a dusty environment, you will not be kicking up so much dust that everyone will be able to point and say, “Over there!” The closed bottom allows you to perform some tuning later on.
The next flash hider to consider is known as the Vortex. The real-deal, actual Vortex is made by Smith Enterprise. It's longer than the A1/A2, and has only three or four (depends on the caliber) open-ended slots. The tines of the Vortex are thicker than the slot walls of the A1/A2, and its slot is machined along a slight spiral, angled path. As a result, the Vortex does a much better job of turbulating the gas flow of the muzzle blast, thus hiding the flash. (Yes, turbulating. It's my book, I get to invent words now and then.)
The one drawback is that the open ends of the tines can catch on stuff like vines, branches or gear.
Everyone from the smallest machine shop to massive offshore manufacturers has made copies of the Vortex design. I’m as much in favor as saving money as the next guy, but when it comes to intellectual property, I’m also interested in protecting it. So buy original if you can.
There are more kinds of flash hiders: ones with slots, drilled holes, ports, vanes, grooves, you name it. All of these are marketed as improvements, but between the A1/A2 and the Vortex, there isn’t a whole lot of improvement to be gained.
How does a muzzle brake work?
A muzzle brake is a simple thing in theory, and a difficult thing in the real world. A brake directs hot, high-pressure gases, turning the gas flow into something we can use.
Newton’s Third Law tells us that for every action there is an opposite and equal reaction. When the bullet begins to move, the rifle begins to recoil. We can calculate the movement force precisely because we know the bullet’s mass and velocity, and we know the rifle’s mass and velocity. The first approximation is muzzle velocity multiplied by mass. For a .223/5.56 with a 55-grain bullet leaving at 3,000 fps, that comes to 165,000 gr. fps. The rifle weighs 7.5 pounds. There are 7,000 grains to a pound. So, our first estimate will be that the rifle will recoil at a velocity of 3.142 fps. (The actual total recoil is derived via calculus, as the bullet does not spend the entire bore time at 3,000 fps. I’ll leave that as an exercise for the really math-happy reader to solve.)
The same estimate, using a hunting rifle in .30-06 with a 180-grain bullet at 2,500 fps out of an 8-pound rifle, gives us 8.035 fps as the recoil velocity. Even if you’ve never fired an AR before, you can clearly see that the recoil it generates isn’t all that onerous.
The muzzle blast is basically a jet nozzle, and we can also calculate that. If we take a basic .223/5.56 load, with 25 grains of powder, and we burn and eject it, we have extra recoil. The hot gases jet out at over 4,000 fps, in some loads as high as 5,000 fps. We’ll use the lower figure. 25 grains (we’ll assume it all ejects at full velocity) at 4,000 fps is an additional 100,000 gr. fps in recoil. That adds 1.90 fps to the recoil velocity of the AR. (The .30-06? It adds 50 grains of powder at 4,000 fps, for an extra 3.57 fps. Ouch.)
But, what if we redirect those gases? A brake can jet them out to the sides or top, or cause them to slam into a fixed barrier, decelerating them. You can use the leverage the barrel provides, and not just negate the added gas component to recoil, but pretty much negate all the recoil. And that’s what a muzzle brake does.
You will notice that there is no mention of turbulence or of reducing flash. That’s because most muzzle brakes are actually flash enhancers. They redirect gases as smoothly and cleanly as possible. A good muzzle brake is loud, as the gases are re-directed back at you, or the guy next to you. They are dangerous, as the 4,000+ fps gases have been smoothly (smooth means efficient, and efficient means no loss of momentum) squirted up or to the sides — or both. You do not want to be standing next to someone shooting a braked rifle in any caliber.
Despite the assurances of manufacturers, there is no such thing as a device that is an effective muzzle brake and flash hider. You have to pick one.
Editor’s Note: This article is an excerpt from Gunsmithing the AR-15: The Bench Manual by Patrick Sweeney.
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