John P. Schenkl's widow was granted a US Patent for one of his inventions on January 17th, 1865. Presumably, he died sometime in 1864. I cannot find any information about the cause of death. I have never heard that Schenkl died due to a projectile or fuze related accident. Where did you read that he did?
There is detailed information about Charles T. James' death (on October 17, 1862) due to the accidental explosion of one of his shells. The detailed info is given on page 191 of the 1993 Edition of the Dickey-&-George book. If you don't have a copy of it, check the less-detailed version at Wikipedia. It says:
"During the demonstration of a projectile at Sag Harbor, Long Island, New York, a worker attempted to remove a cap from a shell. It exploded killing the man and mortally wounding James, who died the next day."
Regards,
Pete