100mil is the thickness of the .999 fine silver plate. The core material (bulf of the weight) is not silver.
mil  a unit of distance equal to 0.001 inch: a "milli-inch," in other words. Mils are used, primarily in the U.S., to express small distances and tolerances in engineering work. One mil is exactly 25.4 microns, just as one inch is exactly 25.4 millimeters. This unit is also called the thou. With the increasing use of metric units in the U.S., many machinists now avoid the use of "mil" because that term is also a handy slang for the millimeter.
Edited by WoodenJefferson (05/12/0803:15 PM) Edit Reason: mil definition
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Loc: East central Indiana
Also be warned that although as WoodenJefferson said a Mil is a thousandth of an inch, these companies that have been making the silver and gold plated replica coins have been using the term Mil to stand for a MILLIONTH of an inch! A true 100 Mil plating would actually be a tenth of an inch thick or 2.5 mm! A very substantial layer! (A copy silver dollar with a true 100 mill silver plating would be over 5 mm thick just from the plating! The maximum size that an NGC slab can hold is 4 mm thick.) A 100 Mil thick plating with these new companies definitions is just a ten thousandth of an inch thick, or half the thickness of the plating of the copper on a zinc cent.
I was aware. These are apparently replicas of LE 1 oz silver bars. Changing the core to base metal while retaining size must have lowered the weight.
Correct. A base metal copy (using copper, nickel or a coppernickel alloy) of a pure silver object will only weigh 85.7% as much. So a copy of a 1 oz silver bar would only weigh 26.66 grams.
Slab collector and researcher
reported as of 2/5/06
150 companies 331 production varieties