I don't believe that the Edward Barnhart killed by the gun accident is the correct Edward.
In the 1900 census for Massena there is the following:
Barnhart, Edward, head, 52, 12/1847, NY, NY, NY, farmer
Barnhart, Keziah, wife, 54, 11/1845, NY, NY, NY
They state they have been married 32 years with 1 child and 1 child living.
If you look at the headstone on www.findagrave.com
it looks like the other name on the stone ends in ZIAH which is most likely Keziah, so this Edward was still alive in 1900.