How Flash Suppressors Work
Robert Silvers, who is head of R&D at AAC and inventor of the AAC Blackout flash suppressor, explains how flash suppressors work, and how his thorough understanding of the science involved shaped the AAC Blackout. Reprinted with his permission.
When designing (the AAC Blackout), I read a bunch of patents and did not find many instances of inventors knowing why flash formed or why their design works. Because of this, they often added in features which did more harm than good. I kept the Blackout efficient by not making those mistakes.
There are three main types of muzzle flash: Primary, intermediate, and secondary. Additionally, there may be an "afterburner" effect. Mechanical flash suppressors, such as the AAC Blackout, have little opportunity to control the part called primary flash (the sparks you sometimes see) because that is powder burning within the barrel. But we can prevent the external flare up which we most associate with muzzle flash - that is the secondary and afterburner effect flash.
In order for flash to occur, fuel, oxidizer, and a source of ignition must be present. The fuel is contained in the smokeless propellant. Ignition comes from heat – but only if an oxidizer is available from either the propellant itself or the surrounding atmosphere.
Typical smokeless propellants contain more fuel than needed to balance with the oxidizer. The mixture is run 'rich' to keep flame temperatures lower so as to reduce barrel throat erosion. This imbalance, while reducing the temperature of the mixture, has a side effect of expelling excess unburned fuel from the barrel. This particulate is what makes secondary flash possible. And as said, if you reduce it - the barrel will burn out sooner.
What is the source of heat for the ignition? Surprisingly, not the burning of the powder - but rather it has to do with the nature of supersonic gas flow. When the bullet exits the muzzle, the discharge of expanding gas is moving at supersonic speeds. The speed of the gas is faster than the bullet itself so it will be supersonic even if the bullet is moving slower than the speed of sound. Because the gas is under such high pressure, it is 'under expanded' when it is released to the environment. This pocket of particles and gas is contained within the shell of the external blast wave and as it expands, it cools. The external environment is pushing back, and the shock wave that forms will reflect this discharge back onto itself into a reverse shock wave known as a Mach disk.
These formations occur any time a flow exists a nozzle at supersonic speeds and at a pressure that is higher than that of the external atmosphere and are sometimes visible behind certain high performance aircraft. It is at this location that the supersonic flow changes to subsonic. When the shock wave passes through the Mach disk, the sudden deceleration and resulting compression greatly raises temperatures and can ignite the remaining fuel, provided there is oxidizer available.
If the oxidizer used during ignition is provided by the propellant, the result is a combustion known as secondary muzzle flash. If the oxidizer is provided by the surrounding atmosphere, it is more precisely described as an afterburning effect. In either case, the result is what we think of as muzzle flash.
The two main ways to suppress flash are by chemical or mechanical means. Flash retardants may be mixed into smokeless propellant to reduce the potential for flash. Generally alkali salts, 0.5 to 5.0 % by weight are used. Flash suppressants are usually not in propellants because they degrade the performance and increase smoke. Military customers often request the chemical additives, but the amount used, in consideration for the negative effects, is likely chosen to be effective only for typical barrel lengths and only most of the time. This means that shorter than normal barrels may find that typical mechanical flash suppressors (such as the A2) – even when combined with chemical flash retardants – are not sufficient to eliminate visible flash.
The AAC Blackout works to reduce pressures and temperatures in the gun muzzle flow field and hence there is also a reduction of strength of the resulting Mach disk. It does this by dividing the expanding flow into several weaker streams. Because of the weaker Mach disk, there is less sudden compression in a concentrated area of the gases as they go from supersonic to subsonic, and so the gas and particulate temperature will stay below the level required to initiate secondary combustion or afterburning.
How Well do Melonite and Nickel Boron (FailZero) Resist Corrosion, Part 2
As I posted yesterday, I recently disassembled the Spike's Tactical 5.45x39 upper receiver assembly and checked it for corrosion. I had intended to post more photos last night, and apologize for not doing so when I said I would.
As noted, the weapon was very dirty. If you are not familiar with my use of this upper, it is important for you to understand that I have fired over 7500 rounds of corrosive ammunition through it, over a period of a year, without one proper cleaning.
The first thing I examined in detail was the barrel. There were no signs of corrosion on or in the barrel, or any other melonited part, for that matter. The gas tube and front sight base were also melonited, along with the handguard cap. Be advised - you can click on the photos for bigger pictures, and when I say bigger, I mean BIGGER. These photos were taken immediately after disassembly, and the barrel was not cleaned prior to taking them. I have run a BoreSnake through it since, and there are no signs of wear or corrosion there, but I'm still working on a decent enough photo of it.
Compare this with the Smith & Wesson 5.45 AR-15 that I put a similar amount of corrosive ammunition through over a similar period of time, also without cleaning. Its barrel was chrome lined, with a phosphate exterior finish. The bolt was originally phosphated, but I electroless nickel plated it after seeing some corrosion - closing the barn door after the horse had escaped, I know.
In comparison, the Spike's Tactical 5.45x39 upper's bolt had been plated with Nickel Boron by FailZero from the start. While it certainly looked better than the competition, there was minor pitting visible once I cleaned away all the carbon. Again, click on the photo for a much larger version.
So it would seem that Nickel Boron is far better than phosphate as a corrosive resistant finish, but is not corrosion proof. Although I would have to have had two otherwise identical parts, one nickel boron and one melonite, subjected to the same treatment for me to conclusively say that melonite offers superior corrosion resistance, the almost-new appearance of the melonite barrel - after over seven 1080 round tins of corrosive 5.45x39 surplus ammo - impresses me.
On the left is a Spike's Tactical nickel boron plated bolt carrier group for 5.56, and on the right is the same in 5.45x39. Both have seen fairly extensive use, but the 5.45x39 version looks just a bit more disheveled.
Metal Finishes and Treatments, Part 2: Melonite and Spray-On Finishes
In the last installment, I talked about the importance of heat treating and anodizing as they relate to AR-15s. The same basic concepts apply elsewhere, too. As before, I consulted experts wherever possible - but if there are any errors, they're my fault.
Today, I'll briefly (okay, not so briefly) discuss two finishes that you might find on an AR, but are more likely to encounter on a handgun: Melonite and spray-on finishes. The latter generally contain the word "Kote" or "Cera" in their name. Or both.
Melonite/Tenifer/Nitriding/Nitrocarburization/QPQ
Known by a variety of terms, this process involves a lot of heat and some interesting chemistry. It can be applied to a variety of steels. There are varying levels of the process, which is broken down into segments called "Quench, Polish, Quench" (hence, "QPQ").
While many of the claims arising from and most of the mystique surrounding Melonite relate(s) to the performance of the full QPQ process, it's possible for only the first "Q" to be done, and stainless steels don't receive the full corrosion resistance benefits of QPQ. This means that QPQ'd stainless steel will have less corrosion resistance than QPQ'd carbon steel. But I'm getting ahead of myself...
Melonite is "a thermochemical treatment for improving surface properties of metal parts. It exhibits predictable and repeatable results in the treating of low and medium carbon steels, alloy steels, stainless and austenitic steels, tool and die steels, cast and sintered iron."
You'll often hear people refer to items being "coated" or "plated" with various processes, with "coated" being the more popular term. In this case, though, it's a big misnomer. Melonite is more correctly referred to as a "treatment." It's not sprayed on like a coating, and it doesn't build up on the surface of the metal like a plating. If you want more details on the process, click the link above. I won't bother paraphrasing further. I'd rather concentrate on what Melonite does.
Melonite offers excellent corrosion resistance, high surface hardness, and increased wear resistance. This does not necessarily mean that it will never show signs of cosmetic wear, or that it will never rust. Even if the process is completed correctly (there have been many instances of improper Melonite treatments, especially on some newer production handguns), rust can and will occur. For example, I have had rust form on a number of Glock slides, which are treated with Tenifer, Glock's version of the process. This rust occurred in the course of normal carry in under one day.
Most people, though, will never see rust on a "Melonited" surface, even in extreme conditions.
As for surface hardness, the number which is generally thrown about is 70 on the Rockwell "C" scale. This is far higher than the 28 or so which most AR-15 barrels are hardened to, at least on the outside - quality carbine barrels are generally hard chromed, which has a higher hardness - but that's a topic for another day. What does this mean for the end user? Well, the exterior finish is going to be much less susceptible to cosmetic damage from rough handling.
Wear resistance? Well, I've heard many things, but still have yet to see some sort of scientific study. Still, when one hears so many positive stories from respected sources, doubt begins to disappear. One highly respected AR expert (and manufacturer) is said to have fired over 50,000 rounds through a nitrided barrel without any signs of excessive wear or that the barrel is anywhere near "shot out." POF reports that they have seen barrels with over 40,000 rounds through them that do not need replacement.
"This sounds great!" you say. "Sign me up! I want my barrel to have this coating -er, treatment!"
Not so fast. Due to the extreme temperatures involved, many parts that have already been heat treated or assembled may not be suitable for nitriding.
AR-15 barrels, for example, should not be nitrided once assembled, because the differences in material between the barrel extension and the barrel result in dimensional changes that can cause the barrel extension to come loose. 1911s with tightly fitted components have come out of the process with parts that are either loose or impossibly tight in relation to one another - and due to the high surface hardness, fitting is very difficult. Not impossible - just very difficult.
However, if you can find nitrided components from a reputable manufacturer, you will likely be very happy with their performance. Why do I specify a "reputable" manufacturer? Because many companies that sell ARs latched on to nitriding as a cheap and easy way to sell parts as being durable and desirable - not knowing that nitriding already heat treated bolt carriers would cause them to anneal (and crack under use), or that nitriding assembled barrels would cause them to come apart after mild use.
Stay away from companies that expect you to do the final testing on their products. Ask them how their nitriding is done, and what quality control procedures they have in place to ensure that the process was done right.
"Kotes"
Gunkote, KG Kote, Duracoat, Cerakote, Cera-Hide. Are they all basically the same, like the various "forms" of nitriding? Actually, they aren't - and the discussion of how they differ can be a bit of a heated subject (especially when Cera-Plate is involved). I'll try to stay away from that as much as I can, but here's a test sheet from an independent lab. It's a comparison between Cerakote, Gunkote and Duracoat, and shows performance in various wear and corrosion tests. Lest you think that the test was biased, the company is willing to provide "test panels" to any third party lab for a comparison with other finishes.
What you need to remember with each of these finishes is that they are all essentially high tech spray paints. They offer good to great corrosion resistance, in part because properly applied "kotes" act as a physical barrier between the metal of the firearm and whatever corrosive agent is attacking it. However, they don't offer increased surface hardness, and while they are fairly wear resistant, even the best of them don't stack up to finishes like nitriding, hard chrome, or properly applied electroless nickel (and its various derivatives).
Like nitriding, the quality of the results depends on who did the work. I once had a rifle and a pistol "Gunkoted" - the work was over budget, far past the stated deadline, and of poor quality. The individual chose to completely remove all the anodizing on the pistol frame before spraying the Gunkote - within 1000 rounds, the "Kote" was completely gone from the frame rails, and bare aluminum exposed.
I have heard similar reports from AR manufacturers - that anodized receivers were completely stripped via media blasting, and certain high wear areas deteriorated rapidly under hard use, even with Cerakote (which seems to be one of the better "Kotes") on the receiver. As a result, they started requiring that all such areas be masked off prior to the beginning of the prep work.
If it sounds like I'm bashing the "Kotes" - I'm not trying to. I am trying to be realistic as to their capabilities. Not every component needs a Rockwell rating of 70 or higher. "Kotes" are a great option when corrosion resistance is needed, but surface hardness and extreme wear resistance is not.
They're also great for making a worn pistol look new again, turning a stainless rifle barrel into a less-observable color, or matching various components - one company's polymer components in "flat dark earth" won't match another company's. But painting the whole thing will make everything match (if that's your bag).
In addition, they're pretty affordable. While hard chrome is often regarded as one of the "ultimate" finishes for a carry gun, along with NP3 (and now "NP3 Plus"), complete handgun refinish jobs can often cost upwards of $300, whereas companies such as Cerakoter.com offer complete handgun packages for as low as $110.
I had most of one of my 1911s refinished by Cerakoter, and I'm very pleased with the results - yes, it's showing wear with use. But the slide, barrel, and other parts haven't shown any signs of corrosion (nor has the electroless nickel plated frame, which I did myself). Corrosion was my major concern - and was a major problem when the pistol had its stock finish. In fact, Kimber's barrels are "in the white" carbon steel - I shouldn't have to go into too much detail about how that ended with horrible amounts of rust. I'll have a full writeup on the Cerakoted 1911 soon.
Part 3?
So, what's next? Well, I want to cover hard chrome, electroless nickel (and similar platings), manganese phosphate, bluing, and a few others. I'm lucky to have a number of extremely knowledgeable people reading my blog, so if I've gotten anything wrong so far, they'll let me know - and I'll post a correction.
AR-15 Malfunction: Extractor Slips Off Case Rim
In an attempt to explain a few of the causes of failures to extract in the AR platform, I made a short video using high speed footage (you can tell that I really like this camera, eh?).
I must apologize for the low quality of the first malfunction video clip. I failed to properly focus the camera. Still, the basic effect is evident.
The video does not exhaustively cover the causes of FTEs, just one of the more common ones.
I plan to create similar (and more in-depth) videos in the future.
[youtube=http://www.youtube.com/watch?v=dPw2BiDkwHE]
Bravo Company vs Ruger: Recoil Comparison Video
[youtube=http://www.youtube.com/watch?v=pQqhR-bUfaU]