Here is what I would do. I am a lawyer, but this post should NOT be taken as legal advice. This is what I personally would just do and am not advocating or suggesting anyone else do the same.
As a general rule, I would say as little as possible anytime for anything where there may be criminal charges levied against me whether or not firearms related, OUI, jaywalking, etc. My mantra, "admit nothing and deny everything."
It is likely that whatever I would say would be hearsay, which may or may not be admissible depending on exactly what I said, how good a lawyer I have, etc. It could be admitted as an admission against interest or an excited utterance depending on the content of the message (which is likely to be recorded). I would assume the worst and concede that everything I say would be admissible. I would probably say
"Someone with a gun was in my house and tried to kill me - send the police now."
I would let 911 ask questions back about people being hurt, etc., and try and answer "yes" or "no" and no more. Keep in mind in the heat of the moment, any rehearsed speech that I would have is likely to be forgotten by me. In an ideal world I might say,
"A person illegally entered my locked and secure residence with a clear and unambiguous intention to cause certain and absolutely imminent and deadly bodily harm to me, which would have resulted in my death. After retrieving my locked, secure and legally held and purchased firearm, I was left with no choice or route to retreat or escape and was forced, through the person's very own actions, to discharge my legally held and purchased firearm to protect myself from the certain and absolutely imminent and deadly bodily harm that would have lead to my death by that person."
But, unless I printed that out and pasted it by the phone, there is just no way I would personally remember all that crap after being shot at or being forced to shoot someone else. And if I did print it out and paste it by the phone, I likely would be questioned as to why it was there in the first place.
In general, saying more than absolutely necessary is just not good in any potential legal proceeding. Your own prior words can only hurt you. This would be true, by the way, whether you were in Mass. or not. Why make the prosecution's case any easier?