Using Bablefish to translate "weapon" from English to German results in "Waffe".
It translates "wehren" as "resist".
And it translates "gewehr" as "rifle".
Not definitive, I know. Maybe we need someone fluent in German. ;)
I've always thought that Gewehr translated to rifle. The German army from the 1870's through WW1 has designated their infantry rifles with models such as Infantry Gewehr Model 71, Gewehr Model 88, and of course the famous Gewehr 98.
It will give you a word - not necessarily the best word, or the etymologically correct word - something with which to make yourself understood (usually.)
What we need is the German equivalent of the O.E.D.