One of my friends who owns a steel fabrication business watched a demonstration of a water jet cutter whilst holidaying over in the eastern states it was cutting through a 300mm thick piece of steel with .1mm accuracy not the stuff to be trifled with.
I was in a prototype/test shop of a multinational company and they had several of those things. If you need to go straight through something they are the way to go and a joy to watch... since the only heat is from the friction of the water cutting the material, I wonder if this is a good choice for sectioning a gun for material properties testing of interior surfaces? I have a Geha I've long wanted to do some hardness testing on and this would be a good way to do it, albeit probably more expensive than machining a cross section.
When I worked at the Sunoco refinery in S. Philly we cut into the sides of the oil/gas tanks for inspections and floating roof repair with this system. Garnet was the cutting medium. Very finicky to work with as the cutting medium likes to clog.