According to Bill's page on Silver Strike Content in my 2013 Price Guide, in 2005 the IGT changed the silver content to approx 0.5 troy ounces. Prior to 2005, the content was supposed to be approx. 0.6, but I have found it to vary as much as from 0.57 to 0.6 depending on the year and the casino.

Also according to Bill's Silver Strike Content page, the brass rim weighs 0.63 troy ounces, so if you TARE that on your scale, you can always come up with the exact weight of the silver.