Here's the plan:
Chamber: 2" SCH 160 black steel with a thin layer of high temperature ceramic cast on the inside to circumvent rusting. The ceramic material is designed to be conductive of heat, so transfer shouldn't be any problem. The fittings will be all threaded, so I can coat them individually. I discovered a graphite based thread sealant rated for 800 degrees Fahrenheit, which I will be using on the threads instead of teflon tape, which would actually melt at the temperatures I'm planning on (almost 700 degrees F).
Valve: Customised 2" 6000psi rated black steel union. The male sealing face will be machined flat, and I'll create a spacer to fit in the female sealing face that will make it flat. I'll use thin annealed aluminum sheet to act as both a gasket and a sealing face. My only worry here is that the ceramic spacer will crack somehow, but the can is 10 pounds of casting compound, so if one breaks eventually I can always make another one. The compounds are on page 3531 of the McMaster-Carr online catalog, and I'm leaning toward the alumina ceramic right now.
Barrel: I don't really know yet. I'm guessing seamless steel or SS pipe, probably 4 feet or so. This should be enough to hit 2000 fps with a golfball. The high speed velocity f*ck ups of GGDT may be messing with this result (does anyone know the speed of sound in 700 degree F steam?), but apparently it could fire in the hypersonic range with a 10g projectile and a 1" bore barrel 12' long. Any suggestions on the barrel that are actually achievable are welcomed.
Gauging: As far as I know, none. Too expensive, vulnerable, complicated, etc...
If anyone sees any flaws in this design, for God's sake, point them out now. I don't want to die before I build a working thermonuclear warhead
