Whether you work eight, ten, or twelve hour shifts you're off-duty more than on. For a variety of reasons you should avoid becoming involved in incidents off-duty. Firearms and tactics trainer Massad Ayoob has pointed out that on duty you have: body armor, a full-size duty pistol with two spare magazines and possibly a back-up gun, access to a shotgun or patrol carbine, and a partner or the ability to call on the radio for assistance. Off-duty it is quite possibly just you and whatever off-duty firearm/ammunition you're carrying on your person. You probably have a cell phone but certainly not the quick response communication that a radio affords.
One of my mentors in the law enforcement survival training arena is retired Detroit P.D. Sgt. Evan Marshall. I always read Evan's Street Smarts columns in the magazine Combat Handguns (going back to the early 1980s). To me, Evan, who had his share of armed encounters while working as a Motor City copper, always had a way of making tactics and survival concepts simple. Evan was always armed off-duty and it paid off for him on more than one occasion. Several incidents that I remember Sgt. Marshall relating while off-duty had nothing to do with work as a police officer but were with subjects that either attempted to assault or attack him while he was in street clothes. Evan would always caution against becoming involved in off-duty encounters whenever possible. He recommended that you be the best witness and call it in for the on duty troops to handle. That advice holds true today as well but you should be as ready off-duty as on to thwart any attempt to attack you.
Mind-Set
The stressors of law enforcement are huge and oftentimes the last thing you want to do is put that off-duty gun on or stay as dialed in off shift as on but you must. When Officer Ken Hammond from the Ogden Utah Police Department was at the Trolley Station Mall treating his wife to a Valentine’s Day dinner in 2007, the last thing that he wanted to do was get involved in a gunfight. But thank God Officer Hammond had his head in survival mode on that day when miscreant Sulejman Talovic entered the mall with a shotgun and a .38 as well as a backpack full of ammo. It was only Officer Hammond's dedication to duty that lead him to investigate the sound of gunfire and to exchange shots with the gunman stopping the suspect's murderous assault.
Being off-duty means that sometimes the trouble comes to you - whether you are ready or not. Being armed as well as mentally prepared puts you at a huge advantage versus being un-armed and caught off-guard.
What to Carry
Sgt. Marshall would advise you that carrying a decent sized pistol with spare ammunition in a holster that is close to your duty mode of carry would be more tactically sound than carrying a small caliber "mouse gun" where you can't get to it. Yes, carrying a decent sized pistol means that your wardrobe is affected and carrying spare ammo is one more thing to lug around. The alternative of a five-shot revolver with no spare ammo means that if you hit with only 20% of your shots, one .38 Spl. hits your suspect (hardly a wise bet...). Quickly blast through those five shots and you're left with a short club or a bad boomerang.
Identifying Yourself
Remember that on-duty officers may be responding to a "man with a gun" call and cannot readily identify you as a good guy. For that reason, anticipate that on-duty coppers may point their pistols at you and give you orders to put your gun down and get face down on the deck. It might be advantageous to have your pistol holstered for this reason but regardless, be ready and follow commands so you don't get shot by the good guys. This is also another reason why you should not pursue when off-duty in plainclothes (or while on-duty in plainclothes - avoid or put out a "plainclothes officers in pursuit" to advise patrol officers).
Have a Plan
Talk to your family about what to expect if an armed encounter happens while you're off-duty. Work through tactics such as: moving away from you (bullets might be coming in your direction); communicating to dispatch who you are and what you're wearing (Ken Hammond's dispatcher wife had to do this); maybe come up with a code word that means trouble; get away and call help. Go over with your family or significant other that when you give instructions such as "Get down!" or "Run!" they shouldn't question you but rather respond immediately.
Transitioning from Off-Duty to On-Duty
When you say the magic words "You're under arrest," or "I'm a police officer" you go from off-duty status to on-duty and have all the same arrest powers that your state law allows while operating in or out of your jurisdiction You should consult with your agency legal advisor to find out specific limitations on out of jurisdiction off-duty enforcement actions. You also must follow agency policies on use of force, reporting and other procedures. This doesn't mean that when you get involved in a drunken brawl off-duty and are loosing you then try to arrest the other half. But if the action is such that you are clearly operating under color of law you have agency legal protection and workman's compensation coverage. Once again, I strongly recommend against off-duty enforcement activity but if trouble finds you or you must act to save another, you do have legal protection.
I've heeded Evan Marshall's advice my entire career and it has helped save my bacon on numerous occasions on duty and off and I've thanked him in person for his contributions to my survival. The lessons were important and I pass them on to you: Avoid if possible; Carry a gun because you never know; Carry enough gun; Carry spare ammo; Talk and Plan with your family about what you'll do and what they should do before it happens and pay attention to what's going on around you. On-duty or off the mission is clear - plan and train for it and then WIN it!