This bushing is used in place of the booster spring to “lock” the booster in the seated position. This allows proper use on fixed barrel firearms such as pistol caliber carbines like the Colt AR15 in 9mm or the Kriss Vector and handguns like the Walther P38 that have fixed barrels. It is not designed to withstand the pressures of rifle calibers like 556 NATO or 7.62×39 as these have significantly higher pressures and the booster could sustain damage if used on these calibers.