Re: wasn't sure where to put this...
I hope you thanked him up and down - pretty nice of your brother and it is a pretty good design that will be fun to shoot for years to come.
Constructive criticism:
Give your brother an anatomy book so he can put the heart in the right place next time.
Also for a hostage shot - have him make the flipper about 1/2 or even 1/3 the size that it is.
The carry handle if it is mild steel - grind it off. I know it sounds like a great idea but the reality of shooting steel at a distance is that you will miss, some of the misses will be close, those that hit the square tube will rip big holes in it that you will later weld buttons in, and those that hit the handle will make for a very sharp hand hold.
Last thought have him consider wider "H" bases for uneven ground. My MGM has an H that is a bit wider than the silhouette itself.
Joe - the bolt is a stop - hit the box, actuates the arm, bolt stops on the shoulder.
Good luck