There is an entry on "Ancestry.com" for one "Robert Bruce McDowell" of Manahawkin Landing, New Jersey, b.1940, d. 2003.
A quick search on "FindaGrave.com" comes up with another three possible matches for the initials "R.B. McDowell"+ date of death in or after 1997:
Robert Bruce McDowell, b. 1928, N.Y., d. 2002, Perth Amboy N.J.
Raymond B. McDowell, b. 1918, d. 1997, Hillsdale IN.
Robert B. McDowell, b. 1927, d. 2006, Nashville TN.
I'd suggest contacting R. Bruce McDowell's publisher; Krause Publishing Co.
Or rather their successor;
https://www.penguinrandomhouse.com/They might have some contact info such as his home-town / -state on file.
Another tip would be to contact "The Winchester Arms Collectors Ass'n":
https://winchestercollector.org/contact/Since McDowell wrote a book on Winchesters, it wouldn't surprise me if they printed an obituary.