Steam gun technology was more-or-less abandoned, after sporadic experimentation, by the mid-1800s, but I wonder if the materials science assumed in CoaDE would make them feasible again.
The basic idea would be to have a HD resistojet but attached to a gun barrel instead of a nozzle. Put a burst disk set for appropriate pressure behind the slug... eyeballing resistojets and conventional guns now it looks to me like this is maybe a 10% advantage at most, so perhaps not worth the complexity? Actually anything interesting here, or no?
(For that matter, given the strength of some of these materials, centrifugal or pneumatic guns may be worth another look.)
If you are trying for any reasonably fast shot speed, a large factor affecting gas-powered guns is the speed of sound of the working gas. Conventional guns do okay at this (hot exhaust gasses). Pneumatic guns do worse (room temp gas cooling off as it expands). Steam probably does okay. Light gas guns (H2, or He if you are afraid of fire) do better.
However, another factor is how often it can fire. Moving propellant charges in with the ammo is pretty well known and rapid. Gasses? I think someone would need to think of something the military hasn't that works better than what they have for it to be competitive. Burst discs are not so great for reloading, valves are either bulky or limit flow rate.