I just made my Mod M&P 15 Sport California compliant by installing a fixed magazine kit put out by Juggernaut Tactical called the Hellfighter CA Mod Kit. Once this kit is installed, no other modification is necessary. You can view the installation and performance on line at jtactical.com/ca.mod. When the kit is installed, the rear take down pin is replaced along with the bolt catch and operation is quite simple. As long as the upper and lower portions of the rifle are together the magazine cannot be removed. The replacement take down pin is pushed which separates the upper and lower portions of the rifle slightly now allowing the the removal of the magazine with a normal magazine button. The kit cost between $60 and $70 and is really easy to install. Here are a couple pictures of the kit installed on my AR.

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Ed