Roommates had no right to search her room. Wonder if she even can sue them? Seems an invasion of privacy at minimum. Guns are legal in MA per much of what is written above. I cannot believe people think you cannot own guns in any state if you don’t follow the law. You can. 2nd Amendment (ownership laws vary-all states all gun possession).
All that said, I can understand why room mates are concerned. They should move if they have fears, anxiety etc. I would. Yes, they should have asked and avoided this situation. Yes, it could be dangerous if the gun is used incorrectly. Yes, they may not agree with her. No, the fact that one roommate lost a friend to gun violence is totally immaterial to the matter at hand. This seems to bring up a good issue, which is adding guns to a lease much like someone would add pets. In fact, it may be that legally one cannot disallow legal gun ownership in an apartment per the 2nd amendment. If someone wants to own a gun and follows the rules they can. What if she was a police officer? No one would think it strange at all that she had a gun ( part of her job). For some people, gun ownership is important. I’m not comfortable with it but it’s protected under the Constitution and has been protected many times.