Shield Arm’s steel mag catch is optimized to work with the Shield Arms S15 magazine. While it remains compatible with plastic OEM magazines, it can cause increased wear on plastic mags with prolonged use, and OEM mags may not drop free. Shield Arm’s does not recommend using their steel mag catch with plastic OEM mags. They recommend steel on steel or plastic on plastic. If you have a first generation aluminum mag catch, it may not work with gen 2 S15 mags. Shield Arm’s recommends upgrading to one of their steel catch offerings.