The fact that the mage just cast fireball or that that guy next to you just shrugged off a hit from a fist the size of an elephant should clue most people in that a change to reality has been noted!
But again, NOT THE POINT. The point is making a ruling contrary to the rules, with no notice, that is harmful to the PC - is not ideal DMing. That's it.
But is it really contrary to the rules?
First off, there's the Massive Damage rules, both the PHB version (if the damage you take takes you down to negative your hit point maximum, your dead, no saving through) and the DMG' addition (taking a certain amount of high damage does something permanent, up to instant death).
Now, regarding the idea of "putting a gun to your head and pulling the trigger" I don't the makers of the rules felt the need to address that. Partially because firearms aren't usually in D&D, but also because
it's blindingly obvious what happens when you do that.
At this point I am seriously leaning towards the opinion that you're seeking to know all the rules so you can game them to your advantage, because otherwise you wouldn't be asking so many things about a situation and resolution that is, again,
blindingly obvious to almost everyone.
Like by daring to play a warlock.
So because he doesn't give you absolute freedom, that means he must hate it?
As I am
very sure you are aware, given how much you cling to them,
there are rules, and if you don't follow them, expect to be punished, and ignorance of the law is no excuse, otherwise you could just do anything with the caveat of "well I didn't know we were ruling that way" despite it, for the third time, being
blindingly obvious.