According to the silver strikers website, the following:

Strike Composition
$2 and $3 strikes are made of Brass.
$7 strikes are .999 Silver with approximate weight of 0.65 troy ounces.
$10 strikes have a .999 Silver center insert (additional info here), outer ring is Brass (additional info here), approximate Silver weight is 0.60 troy ounces of Silver. Sometime in 2005, a new thinner center insert strikes with less silver (~ 0.50) appeared. In 2009 CLAD (no .999) $10 strikes appeared, even some red capsules. However, the red capsules should return to .999 silver.
$20 strikes have a .999 Silver outer ring with some 24kt Heavy Electroplating (HE) and a Bronze 24kt Heavy Gold Electroplate (HGE) on the center insert, approximate Silver weight is 0.75 troy ounces of Silver.
$28 strikes are .999 Silver with 24kt Gold Plating (GP), approximate Silver weight is 0.65 troy ounces.
$40 strikes are .999 Silver with 24K Heavy Electroplate (HE), approximate Silver weight is 1.54 troy ounces for the large size $40 strikes and 1.26 troy ounces for the small size $40 strikes.
$200 strikes are 1 troy Pound (12 ounces) of Silver and may have some 24kt Heavy Electroplating or for the newer RWM mint strikes may have colorization.
$300 strikes are 1 troy Pound (12 ounces) of Silver and may have some 24kt Heavy Electroplating.  Beginning 2011 the new $300's were changed to a 6 troy ounces center with a gold plated copper