I've heard of the silverado causing skin burns for some people. the problem seems to be the pads are too close, or the current is not properly limited. the closer the pads are together, the more current can go straight along it instead of spreading through the body.
what you can try is touching the pads with different hands, or perhaps finding a thicker spot of skin to zap. (zapping doesn't need high current to work.) make sure the skin between the pads isn't moist.