Excellent mags for almost every M4 variant . The only weapons that seem to have issues with these mags are KWA M4's . This is due to the 2 piece hop-up used by KWA . The springs in these mags are very strong and will push the bbs up against a small shoulder in the top portion of the hop-up , resulting in a failure to feed . If you have a KWA , it's worth modifying the magwell and replacing the air nozzle so allow installation of a 1 piece hop-up unit (and an Ajax Customs STS!) . This will allow the mags to feed as flawlessly in KWA's as any other M4/16 . The steel used on the springs is tempered , resulting in a much longer lasting spring , and allows it be stiffer than most mag springs , which results in VERY reliable feeding , regardless of ROF . If you run a LiPo or have a high speed motor for high ROF , these are the mags for you!