I get letters daily about how to do this, on every gun you could imagine. I've posted a few how-to's but never in this detail.
So if you have an MP7 that really, really, needs a huge boxmag, you will have an idea of how to get started.
I've built literally dozens of these at a time, and they work.
And you can do it for about twenty bucks.
Rather than load a huge string of images here, have a link:
Before anybody asks, NO I do not sell what you see there.
http://www.air-sharp.com/magmod.htmlA boxmag for an MP7, using a micro servo.