How do I make a symbol for "average molecular weight"?

I need to make a symbol for "avaerge molecular weight" and save it as a
symbol that will be used frequently. The symbol looks like Mw, but with a
line above the M. I have been trying to use Insert Equation, but it is not
always highlited in documents and I am having difficulty using it.

Thanks for any information!!

See [URL="http://sbarnhill.mvps.org/WordFAQs/Overbar.htm"]http://sbarnhill.mvps.org/WordFAQs/Overbar.htm[/URL]