Gun is fine, the problem is ammunition or any ammunition component including black powder.
Make your life easy and apply for a FID card and CYA, it isn't worth the legal fees if some cop or nature cop decides to cuff and stuff you for the courts to sort out.
I just want to repeat myself from post 13, now that people who know more than I have chimed in
You have had a couple of people chime in that have passed the bar ( do not take anything posted on this forum as legal advice btw)
They have given you their personal opinions
Others who are pretty knowledgeable have chimed in.
It seems to me POSSESSION of certain BP is legal without any permit or license, it is BUYING them that is an issue.
Even then any FFL in MA can say no to selling you BP and probably will.
So after several days of discussion, the 2A friendly folks have given you opinion
Now you are out in a field, it could even be on your own land, or a friends land, or a private property where you have written permission to be.
You are trying to take some sort of game, fowl, or otherwise.
You let off a couple of blasts and one of the Karen's down the street calls the PoPo and they roll up to investigate.
Cops are not Lawyers, the chances of you having a Cop with deep knowledge of MGL on BP hunting and what you are legally able to do is near zero
What you are going to get is some cop who is going to disarm you at gun point, and then arrest you because all he or she knows is they were taught a FID is needed for a rifle, you don't have one, and they are going to get lots of OT for going to court from the arraignment thru the trial, because you will prosecuted and it will go to trial.
You will spend tens of thousands of dollars on legal fees, time out of work, etc etc etc.
Now I am not a fan of MA or their gun laws, there is a reason I don't live their anymore, but for the cost of a FID 99% of your risk can be avoided.
And you can go buy as much BP as you want from your friendly local gun shop
If you want to test the system, go wild.
This is not the hill I would want want to die on.
Oh and IF you prevail, good luck getting the rifle back from the cops without a court order, assuming thy have not sent it to a bonded warehouse where it will cost you thousands of dollars to get it back