Simply the best i have used - buy it. - ODIN Works Clamp-On Adjustable Low Profile Gas Block

If you need an adjustable gas block, you might be tempted by cheaper options. Don't be. A component like this isn't a place to cut corners. I am a penny-pincher and love a good deal, and am convinced this is a great value _despite_ the seemingly high price. I have three Odin adjustable gas blocks and they all installed easily and function flawlessly. This clamp-on model might not fit under the really small-diameter free float guards, but the set screw model likely will (I have both types). I personally like the clamp-on type for strength and security, as well as the fact that it can be indexed any way you want on the barrel, unlike when the "dimples" on the barrel aren't in exactly the right spot and cant the gas block...you know what I mean :/

As for why you might want an adjustable gas block...well, I can tell you what it did for me: In a 7.5" .300BO pistol it allows me to vary the gas pressure at will to ensure that I had minimal recoil but full reliability with both supersonic and subsonic loads...you simply adjust the block when you switch ammo. Tip: keep the allen keys for the gas block in a tiny ziploc bag inside the storage compartment of the weapon's grip ;)

And in 16" heavy-barreled .223 Wylde carbine I used it to trim the gas pressure down, as a carbine-length gas system with a 16" barrel tends to be over-gassed and the recoil a little punchy. No need to have that carrier group slamming any harder than required for function, eh? And with a Stag arms 3G comp venting muzzle gas upward, that carbine not only gives minimal recoil, but zero muzzle rise. For real. Zero.

And in a 20" .223 Wylde medium-weight rifle I used a Odin adjustable block to ensure there's just enough gas coming into the system to cycle reliably using a pneumatic recoil system (that's right, not a pneumatic buffer, but an entirely pneumatic recoil system: no weights chunking back and forth under spring pressure, just a low-mass gas piston). With a big side-vented brake, tuned gas system, and that pneumatic buffer, it's the least recoil I've felt in a .223/556, including my big heavy target rigs.

So obviously not every AR needs an adjustable gas block, but some projects, especially ones that stray significantly from Stoner's original design, can benefit hugely from one - and the Odin is my favorite. It's surprising how much more enjoyable an AR is to shoot when it's not over-gassed!