Duds are an interesting subject.
When is a dud a dud ? If it doesn't go off when the launcher wants it to or ever ?
The categories are almost endless, and no doubt separated so no true number/% can be determined ...
Torpedoes
Bombs
Artillery
Mortars
Grenades
mines
Smaller mm 20/25/30/40
I guess it's something no one really wants to remember/talk about/clean up. Just turn the place into a cow pasture/fish farm and it's taken care of.
I've heard as much as 30% in sand/marshes, and early American torpedo's litter the ocean floor. Early VTs were also prone to malfunction as well as late German arty.
can't transfer the other link...it was
http://maic.jmu.edu/journal/4.2/Features/ww2.htm
The Mine Action Information Center
at James Madison University
Article called
WW II Ordnance Still Haunts Europe and the Asia Pacific Rim...by Margaret Buse'
or
Battlefront.com Discussion Forums posted by "moosehead" Jan 31st 2002