Pretty much covered in the post above although I will add that the 6721 has a 1/9 barrel twist while the other two models mentioned have a 1/7 twist.
"I would have done it right the first time, cept I was in a hurry." "I forgot to tell you because I was drunk." "Drug test, dude, I think I smoked some weed last week. I can't remember because I had such a good time."