Keeps the inner barrel perfectly centered in the chamber, no more play with the screws and adjusting by eye. Just add this, LIGHTLY tighten both screws and it's done.
It will also help to prevent the overtightening of screws and protect inner barrels by keeping the barrel at equal distance from the walls of the chamber.
Fits stock barrel, PDI, ZCI and all good quality other brands.
**Only for use with the Standard Hop Unit - NOT the Fast Hop Unit**