A castle nut has a groove around an exterior circumference thereof. An end plate has a through hole extending in a direction perpendicular to a longitudinal axis of a firearm. When a buffer tube is threaded onto a lower receiver of the firearm with the buffer tube traversing through the castle nut and the end plate, the castle nut can be screwed in a direction toward the lower receiver to fasten the end plate against the lower receiver. When the end plate is fastened against the lower receiver by the castle nut, the securing device can be screwed into the through hole of the end plate such that a portion of the securing device is engaged with the groove of the castle nut to prevent the castle nut to be loosened by being unscrewed in a direction away from the lower receiver.