Adam, you'll get as many different opinions as you get replies on this one.
I like to base coat with flat black, then wash with a burnt sienna and tan mixture for rust (if appropriate), then dry brush with Testor's Steel and highlight with Testor's silver. But I've also used Vallejo and Tamiya Gun Metal. All seem to work well.
Good luck.
Bill
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DML M4A2 Red Army
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-- There is a fine line between "hobby" and "mental illness". (Author unknown)