Black guy here...
I know many of you mean well, but I think we should stop perpetuating the notion that "nigger" is offensive, but "nigga" is an acceptable term of endearment.
In the black community, it is used as a term of endearment. It's also used as an insult. It's often used as an empty pronoun in reference to any person, regardless of their race or ethnicity. Basically, the word is used so liberally, that the meaning behind it is purely contextual.
But it's important that you note I said "in the black community". If you are outside of the black community, chances are that if a black person that hears you using any pronounciation of it, he/she will be offended. They may or may not make this known, but this is the reality. Also, there are many black people who find the word offensive, regardless of who says it. Finally, there are people who aren't black who feel the same way.
I just don't want anyone finding themselves in an unneccisary altercation, because the Internet told them "nigga" was a-ok.