I worked as an Administrative Assistant in a Police Station in London for 2 years. I can assure you that most criminals are not rational. The moment Police appear with guns, it justifies criminals using theirs to their eyes. Police would never consider carrying knives because they consider them barbaric, however, pointing a tube at someone and being able to execute them without trial is ok.
A lot of the people who were arrested at Islington for possesion of weapons were often so high on drugs that they were not even aware that they could be killed, or possibly even that they'd been arrested, they would spit, bite and make murder threats to all and sundry all the way between the vans and the holding cells.
I'm not talking about those who choose not to arm themselves, I'm talking about those who do arm themselves, and will see the armament of the Police as justification.
Shootouts between gangs and Police will happen because it is 'cool', it happens in the movies, as much as people argue about this, I believe most people live in a mental 'movie' in which they are the star. It's all about 'having the right attitude', 'adopting the right pose' and 'Getting Respek' If you have bragging rights of a shootout with the Police, it adds social 'weight' to your reputation. Whilst the UK does have it's run-down and poor areas, I personally feel this is a lot more about image than crime in desperation. There are people who no more see Police officers as human beings as the members of the public they commit crimes against.
That is why I think it would happen, most people aren't actually aware that they are mortal until the moment before they die.