According to R.S. Yeoman's The Official Red Book, the
composition of U.S. half dollars is the same going back as far
as the Barber (or Liberty Head) half dollars. The
1873-1874 issues of the Seated Liberty half dollars (Variety 5)
have "arrows placed at the date to denote a change in weight
from 12.44 to 12.50 grams."
Weight: 12.50 grams
Diameter: 30.6 mm
The 1964 half dollar's composition:
.900 silver, .100 copper
Net weight in pure silver: .36169 oz.