The gun laws in Massachusetts are extraordinarily complex. There are several factors to consider in your question. First, you absolutely cannot carry ANY gun into a public place (a bar for instance) unless you have either; a Firearms Identification Card (FID), or you have License to Carry (LIC). (*There is a whole separate section of Massachusetts law dealing with fully automatic weapons, but I’m assuming you aren’t referring to those.)
Unless you are a police officer, work for the government, or hunting, you can never carry a loaded rifle or shotgun anywhere on a public way. If the rifle or shotgun is unloaded, you may carry but the weapon must be in a case (there is an exception if you’re participating in the reenactment of a historical battle). I told you this was complex.
The laws go on and on. However, I would advise that even you have a license or FID card, do not carry even an unloaded firearm in a bar or restaurant. Massachusetts allows people to store guns in their cars as long as it is in a lock box. Carrying a loaded weapon in a restaurant or bar (even with a license) in Massachusetts is not a good idea.