Technology is great when it works... When it doesn't
Here's a good slow motion video of the stock AK slant century sends on VZ2008s:
http://youtu.be/kXtO7KLip9M?t=11m5sEspecially visible when he's shooting w/o the stock:
http://youtu.be/kXtO7KLip9M?t=12m1sAs you can see it pushes the muzzle down and to the left... For AKs, it counteracts the movement of the rifle due to the cycling of its off balanced and heavy bolt carrier to help keep the rifle on target. Far from ideal for the VZ58/2008. Century included it as they just needed to rethread an item they already had on hand for 922r and was probably cheaper than buying Czech muzzle nuts (the next cheapest option).
Flash hiders are relatively straight forward -- their purpose is to disperse and cool the gases leaving the barrel, making combustion less likely and less concentrated. While sometimes they do change weapon performance, the effect overall is minor. FWIW, the combustion we're talking about is 2 fold -- 1) gases from the bore (primarily hydrogen and carbon monoxide) mixing with air (primarily oxygen) and causing a chemical reaction that causes flash and 2) unburnt powder being seen as sparks. By cooling the barrel gases and reducing the amount of barrel gas/air interacting in any specific place (basic chemistry, heat speeds up chemical reactions), the visible signature of these chemical reactions is reduced -- BUT, it's primarily reduced in the visible spectrum and would still show up on most night vision devices...
Good read on the causes of flash:
http://legallyarmedamerica.com/2014/01/10/muzzle-flash-what-is-it-and-what-causes-it/Flash through the eye and 3rd Gen NVG (fsc does a good job w/ barrel gases, small red glow, but out of that 14.5" barrel, there is quite a bit of unburnt powder -- the flying sparks, so good for illustrating those components too ; first shot from suppressor, there's still quite a bit of oxygen/normal air in the suppressor and until that is displaced by muzzle gases, there is increased flash and noise):
Brakes work by redirecting gases in such a way that it changes how the weapon recoils. Without a brake, it recoils straight back -- that is, the gases leaving the barrel are propelled largely forward and that pushes the rifle straight back (think like blowing up a balloon); there are other recoil forces like bolt carrier, etc, that are another issue...
With a brake, there is really only a hole for the projectile and that hole is usually less than 1/3rd the total front are of the brake. As gases leave the barrel, they immediately expand and follow the path of least resistance. So the closer brake features are to the bore crown, the greater impact they will have on the brake -- greatest effect in location is seen w/ multiple chambers but its relevant on single chamber brakes as well.
Also, almost all brakes help to lengthen the recoil impulse which makes it more controllable.
Baffle type brakes send gas backwards (others will move away from you on the firing line as noise follows the gases...). The Special Forces brake side ports are directed backwards as well. This function reduces recoil the most as it effectively pulls the rifle forward, rather than just preventing gas force from pushing the rifle straight back, what other brakes do.
Generally, lateral ports serve to promote muzzle stability, while reducing recoil less than backwards facing baffles/ports. Upper ports help to combat muzzle rise. And the rest of the features follow these basics. Generally, muzzle rise is needed most when taking rapid shots, while muzzle stability and recoil reduction. Generally, brakes are biased in one way or another, but most combine several features. The muzzle will always move in the direction opposite of ported. Ports may cancel out (that's "stabilization" with brakes), and it's relatively easy to tell by looking at them... For instance, the slovak brake is biased to reduce muzzle rise (it's designed around full auto, and the upwards ports come first, and the area of the side ports is much smaller than than other types of VZ brakes -- fwiw, the slovak type brakes do seem to have smaller lateral port holes than most of the other modern Czech manufactured brakes which makes this muzzle rise biased even more pronounced...
Beyond performance of the brake itself, weight is also a concern. Heavier brakes result in a more controllable muzzle when firing but also make for a heavier and slower pointing weapon. Heavier brakes are also generally longer, which allows for a more customized/tailored recoil impulse. So there's a tradeoff like anything...
Related, special forces works as a flash hider largely by having an inner and outer chamber that helps to disperse and cool the gases (through flash hider portion) before sending through brake ports -- the reason most brakes have greater flashes is that they concentrate barrel gases resulting in more heat/flash. And a lot of the time those gases are directed upwards to help w/ muzzle rise, so the shooter sees them as well.
Slant brakes are a little different than others -- they work by preventing the muzzle gases from expanding in a certain direction and overall have flash signatures similar to bare barrel... So w/ the VZ specific slants, they have the protrusion at 6 oclock which pushes more gases to the side and up with the bias towards up, so it reduces muzzle rise.
*The standard A2 birdcage flash hider w/ solid bottom works similarly to slant, but top birdcage does help to reduce flash, especially for the shooter...
Bottom line, perfection with muzzle brakes is very much in the eye of the beholder...
Essentially, similar discussion as here but for AR:
