UTG barrel droop rail.

I have used one. As fas a difference, that depends on a few things. The idea of it it that you can compensate for droop by changing the angle of the scope mount. While I didnt have droop problems, it did allow me to optically center the elevation of the scope and adjust said elevation with the mount instead of the turrets, which allowed for more adjustment left in the scope.

If you dont make any adjustments to it you wont see a differnce, but when adjusted correctly it can mean the differnce betweek a properly set up scope, and maxed out elevation knobs which can cause POI issues due to spring bind / slack in the scope.