Now to assemble the mag. You'll need to scavenge a mag spring from a broken bb gun mag. I realise this is not commonly found, so in my next design the mag will use a rubber band which is much more easily available. You'll also need a few bobby pins, one of which you'll need to cut down to fit the grove near the top of the right side of the mag housing.

The humble bobby pin is actually made from high tensile spring steel, and makes a great spring for a mag catch, or for any other mechanical parts with a relatively short range of motion.



The function of the mag catches are to hold the mag into the mag well, which can then be pushed in, releasing the mag, by the mag release buttons on either side of the lower receiver.



Another catch sits up the top of the mag housing beside the channel for the bb's. This bb catch holds the bb's in while the mag is being loaded. When it is pushed into the mag well, it is pushed back out of the way by the feed tube that protrudes down about 5mm, allowing the bb's to jump up into the hop unit. This accounts for the satisfying click you'll hear as the mag is pushed in.



Unfortunately the gap between the top of the mag and the chamber means about 4 bb's fall out when the mag is removed, so the mag capacity is actually 15, but 4 don't make it into the chamber unless you tip the gun upside down as you cock it. But this is a minor problem most airsofters will have come across before. In my next design however, I will fix this issue with a more realistic loading style where the bolt carries a round up into the chamber, hence covering the vertical gap.

EDIT 06-10-18:

I've uploaded a modified mag that uses a rubber band rather than a mag spring as they can be difficult to obtain. Check out the zip file named 'Magazine Update- rubber band' which includes all the .STL files for printing, a few images to help you assemble it, and a .STEP of the whole assembly for those wanting to take a look inside- questions are welcome!