The finished gun (image 1) has a few things I didn't mention. The little black bit, or a stub to you hard-core k'nex gun makers, from the front view ...
Small add-ons to KILLERK's Magnum that make it stronger, easier to use and perform better. Just a few small changes that make a big difference.
Step 1: Second Handle
This handle, like all of these add-ons, is optional, though I recommend all of these, this is the least nececarry to the gun's performance. Build the piece below (image 1) then attach it to the second set of grey / yellows to the front (image 3).