The Maglock cap serves as a blast gate with a maglock connection. 

The problem with some blast gates, is that they are not effective enough, and can come loose, rattle or lose efficiency over time. To get a quality one you have to spend over $100, granted that one has auto open/close functionality, but the Maglock Cap will be just as effective, but without the automation for a fraction of the price. 

To assemble take the cap and glue it into the mag as shown. Available in 4" and 2.5" sizes.