The problem is unless you have very stringent gun safety or training, carrying a firearm is always going to be trouble. Its impossible to prove self defense when you're armed with any sort of firearm. Because your life is not in immediate danger. And if Mike were to carry around a firearm, and his boss suddenly went missing, well, police might construe Mike was preparing to kill his boss. The gun would cause more trouble then it would solve.
To put it another way:
Bob comes at Dave, Bob is carrying a knife.
In a struggle, Dave is able to disarm Bob and stabs him.
Dave then claims self defense, but is arrested, tried, found guilty and sent to prison.
Why?
Because you can only claim self defense if you believe your life is in immediate danger. In the above example, the instant Dave disarmed Bob, he was no longer in immediate danger and instead was presenting as dangerous towards Bob. Self defense would only be allowed if in the struggle, Dave was able to turn Bob's grasp towards himself and forced Bob to stab himself.