If you have to use one, this works great. - Juggernaut Tactical California Compliant AR-15 Magazine Locking Kit

First of all I have to say that I only install this on my rifles under duress. If you live in a communist stinkhole of a state, you have to use a device such as this, and I've tried a few and settled on this one. If it was something that added a desired feature, I would say that it was a decent value (they always stick it to you when they know it's something you have to have), and that it was easy to install. Buy one if you can't afford to move somewhere that actually respects your God-given rights as a human.