Consider that the energy output of a 12-gauge shotgun is approximately 4500 Newton-meters and, from personal experience, can rotate a first-gen iPad at an extrapolated 240 rpm (extrapolated as this proved difficult to sustain). That gives us an equivalent of 113 kW! A modern ipad would only need about 13 kW to charge in one second.
So, one shotgun shell could easily charge yours and 7 of your friends ipads instantaneously, although the results are difficult to appreciate.