I've been around guns for over 50 years, but never heard of .31 Jap either. Maybe that's because I travel in different circles. When you really think about it the important thing is that when describing anything that the people you're describing it to understand what you are talking about.
Let's face it, when describing poop, or doo-doo, or crap, we know what's being descibed is still just plain sh^t. There's nothing wrong when someone with more knowledge, who means well, shares some of his wisdom by correcting a term being used. Let's face it, this is just a forum and not a classroom, so there's no reason for anyone to get upset about being corrected.
Even those who think they know everything will learn something new just by reading this, as well as other forums each day. By the way, does anyone know what the original ammo was?