The Brownells AR-15 magazine lip gauge serves as an efficient Go/No-Go tool, enabling users to swiftly and accurately verify the correct feed lip gap on .223 Cal/5.56x45mm magazines. This verification is crucial for ensuring reliable feeding and preventing weapon malfunctions due to feed issues. To use, simply position the gauge over the back of the suspect magazine and gently press it between the magazine feed lips. If the gauge passes through the feed lips, the magazine should either be discarded or adjusted using the Brownells magazine feed lip tool, which is sold separately, to restore the feed lips to specification and ensure reliable feeding. Conversely, if the gauge does not pass through the lips, the gap is too narrow and requires widening. The Brownells AR-15 magazine lip gauge is constructed from 4140 chrome-moly steel, heat-treated to a hardness of Rc 50-55, ensuring dimensional stability and resistance to wear and corrosion.