Massachusetts Castle Doctrine...

Chapter 278: Section 8A. Killing or injuring a person unlawfully in a dwelling; defense

Section 8A. In the prosecution of a person who is an occupant of a dwelling charged with killing or injuring one who was unlawfully in said dwelling, it shall be a defense that the occupant was in his dwelling at the time of the offense and that he acted in the reasonable belief that the person unlawfully in said dwelling was about to inflict great bodily injury or death upon said occupant or upon another person lawfully in said dwelling, and that said occupant used reasonable means to defend himself or such other person lawfully in said dwelling. There shall be no duty on said occupant to retreat from such person unlawfully in said dwelling.

As a non-lawyer, I don't  consider this much of a  protection.  It  provides  neither immunity from prosecution nor an assumption that an intruder intends  to seriously injure or kill the lawful occupants.  You could spend  hundreds of thousands of dollars and many years trying to prove your innocence in a liberal state that frowns on gun owners.  It's nice to know that I have no duty to retreat, but tactically and legally, leading my family outside to safety (if possible) is probably the best course of action.

The NRA's Three Rules...

1. ALWAYS keep the gun pointed in a safe direction.
This is the primary rule of gun safety. A safe direction means that the gun is pointed so that even if it were to go off it would not cause injury or damage. The key to this rule is to control where the muzzle or front end of the barrel is pointed at all times. Common sense dictates the safest direction, depending on different circumstances.

2. ALWAYS keep your finger off the trigger until ready to shoot.
When holding a gun, rest your finger on the trigger guard or along the side of the gun. Until you are actually ready to fire, do not touch the trigger.

3. ALWAYS keep the gun unloaded until ready to use.
Whenever you pick up a gun, immediately engage the safety device if possible, and, if the gun has a magazine, remove it before opening the action and looking into the chamber(s) which should be clear of ammunition. If you do not know how to open the action or inspect the chamber(s), leave the gun alone and get help from someone who does.

Other Rules and Considerations

4.    Know your target and what's behind it.

5.    Wear Appropriate Ear and Eye Protection.

6.    A  ball cap will shield your face from flying brass.

7.    Avoid loose clothing that can catch hot brass.

6.    No alcohol, drugs, or prescription medication that make you less alert.

7.    Anyone can yell cease fire at any time.

8.    Keep firearms locked away from unauthorized people.

9.    No ammo allowed in the same room where you are working on your gun.

10.  Keep your hands away from openings and moving parts.

11.  Read your owner's manual.

12.  Keep ammunition in the original box. Use only the correct ammo.     

13.  Treat air-soft and BB guns as if they were real guns.

14.  The safety is a mechanical device which can FAIL.  Don't rely on it. exclusively.  Safety is also a state of mind.