Have tried to research your BARNHART information butwithout a few more details, the closest I've been able to get to your grandfather, possibly, is a listing for Paul Barnhart in the Social Security files.
This number_ 168-18-2660_ revealed only one record with both Pennsylvania and California in it.The number also reveals that it was issued in Western Pennsylvania which is also where Butler, PA is located.There are many, many Barnharts in that area and they are among the original pioneers from the late 1700's.
The S/S record states that Paul Barnhart was born in Pennsylvania on August 22, 1922 and died at the age of 74 on January 1, 1996 in Cardiff, California which is about 25 miles north of San Diego.Cannot confirm at this time that this is your grandfather.
If there are any more details_ slight remembrances or stories related long ago that give some direction_ anything would help.
As a direct family member, you may also be able to obtain WW II military service records for your grandfather through this site:
There are many other sites to search for WW II records but the National Archives is the most thorough.
If your grandfather is listed there, you can request a copy of his service record/discharge papers and posssibly find the rest of your American side in California and Pennsylvania when you have more detailed information.
More than happy to help when you connect to the right branch of Barnharts.They have quite a colorful and industrious history in Pennsylvania and all points West.