It's nice work; this had always been in the back of my mind but the metalwork kind of turned me off. It's a pretty... motivated mod. I mean you know it's srs bizness when the Tactical Tennis Ball comes out.
If he's planning on doing this on a larger scale, I think it would be better to just suck the magazine receiver into a custom integrated rail. The receiver geometries are not that difficult to replicate.
This is just a quick off-the-cuff mockup.
Attachment 95945
You can also easily do other stuff like add “tactical” rails, fidget spinners, swappable trigger guards, etc. You're not just limited to Dye droppings.
FN303's are no good for front players.
Imagine that instead of running off break immediately to the furthest back bunker and sitting there the entire game, a player rushes off break and squats some prime real estate at the 50 armed with FSRs.