So this is my first build, starting from a very cheap donor gun (very cheap due to it's pinkness) and home-brewing or ebaying the various parts:
Before

After

(The John Wick style pistol grip stock was an ebay special)
Now the scary bit - taking an angle grinder to your new gun to open out the loading port (as you would modify an engine head for flow)
Before

After


Note I've also welded and extended the lifter for smoother operation and to not trap my thumb when I'm rushing.
And here is my home made charging handle (high tensile M6 bolt on the lathe, plus a £2 kitchen door knob!)

There are a few internal mods (weakened carrier latch and release button, softer mag spring) but they aren't very photogenic!
The final piece of the puzzle is to increase the mag size from 2+1 to 10+1, but I'm waiting for my little piece of paper from the police to let me do that without getting into a world of trouble.