Police believe he leaned over the device to ignite it and didn't get out of the way before an explosive shot out of the launcher. His left arm was severed at the shoulder.
Smith was taken to Southside Hospital in serious condition while friends packed the lost limb in ice to try to save it. Police believe doctors will try to reattach his arm.
A hospital spokesman didn't immediately return a call.
Police say another person at the party also was taken to a hospital, but it's unclear why.