Adjustable or fixed? Since I have to be difficult I run hand loads with a silencer. So adjustable, obviously. There's several ways to adjust the gas system. A screw is most common. But I like the aftermarket 4 position gas block adjustment on my sagia S12. My AR15 ammo is almost as diverse as shot gun ammo. So being able to know the setting for factory ammo and the setting for 223 start load ammo I reloaded would be nice and not having to hunt for the setting like on a screw adjustable block would be nice. There's 3 ways to attach them. Set screwed, hands down the worst option, when the set screws come loose you're screwed and they always do. Clamp on, a less worse option than set screw. The mini14 gas block is clamp on and I never had one come loose so it can't suck that much. Staked, the way St. Stoner intended. What's the big deal, it's not that difficult to stake an A-pillar. If you can hit your thumb with a hammer you can stake a gas block. It seems everyone avoids staking a gas block like the plague. The closest I can get and keep staked would be an A-pillar cut it way down, with a drilled and tapped gas screw. But if I give up on staked and will accept clamp on it looks like strike industries makes a collar click adjustable clamp on gas block. Appears to be the least worst, most best option. But the way it clamps on kind of sucks.