I am a native Japanese so forgive my poor English.
I think you guys better not to use the word "Jap". It is well-known as a derogatory meanings like Emma says.
What I know is that the word was used during the WW2 in American Army express Japanese, not only Japanese army, but also indicated each ordinally Japanese person.
(I am sorry about the history...)
I do not know any other good abbribiation for Japanese.
"JPN" is better perhaps?