Step 17: Release pin modification
I rotated the release pin 90 degrees so that now instead of drawing back like a bolt, it's hinged and rolls back off the hook of the arm. You launch by pulling back on the lever.
Very simple and you can use the original pin, though it may need some reshaping. Just make sure that it's placed far enough foward to release the hook and that it doesn't snag the ammo on it's journey past.