Just done some further reading:
Gamma Rays are extremely penetrating, the smaller the wavelength (higher energy) the greater the penetration power.
So practically this means that although the positrons and electrons released by the initial hits will 'only' cause havoc in their own vicinity (initially, it is a 'chain reaction' until the energy is exhausted) the gamma ray laser itself and any new gamma rays generated by the hit can travel quite deeply into surrounding materials before interacting. This means that the graser doesn't necessarily have to burn through something layer by layer - it can be hitting the wall, the contents/inhabitants of the room, the opposite wall, the opposite room and it's contents at the same time etc. This also applies to the gamma rays released by any matter that it hits - these may travel quite deeply before they release their energy as well. This means that the radiation spreads even faster through nearby parts of the ship than I previously thought. 100% turned into vapour/plasma is getting more and more likely at this stage.
