Bullets are not the main cause of a loc explosion.
Remember that the steam of a loc is caused through heatening the water inside of the boiler.
The water tries to expand, but can only through the vent for its cylinder.
You have 170-210°C by 12-16 bar inside the boiler.
Now you have a A/C firing on it. The bullets only put some holes in it.
But what cause the explosion is, that the hot pressurized water now can depressure to the point where it becomes steam.
And that steam begins to expand so fast, that it rips everyting apart.
For those who don't believe in science or math (there's no nobel prize for math) and only in moving pictures, there was an episode of "Mythbusters" where they overheated a normal house boiler. It went through the roof.