Take a look at this video about the impact of these loose filled minimum impact rounds, They are designed to displace the energy because they are filled with small balls. Almost like the idea of a riot bean bag displacing energy.
http://www.youtube.com/watch?v=XZoUOwYNU8Y
Now imagine some of those empty shells you can fill yourself with what ever you want.
Ok these FN303 personal protection marker are working at 400fps and normal markers are set at about 300fps i think?
In the USA you can buy one off the shelf, some states make you go on a one day safety course first.
But you can upgrade them, Or have the parts to tune them up if things were going belly up. Once the SHTF.
Not everyone can get a shotgun.
But there are no real restriction on owning a paintball marker so they might be the second best option.
This one is an interesting watch as well.
http://www.youtube.com/watch?v=ld9RIaD3YIw