I am not a fire expert, but I had a 3 week training in puting out fire as I used to be a sailor and actually had to use it twice in real time life threathening situations.
Some notions:
Fire triangle: to burn a fire need 3 things :
1-A combustible (fuel, wood, paper, smokless powder)
2-An oxidizing agent (Oxygen)
3-Heat
A fire spread because the heat form the original fire turn more and more material into combustible. That mean the heat from the fire will turn otherwise inert material into combustible (by heating it and thus turning it in gaz) and thus feed the fire wich will grow bigger and hotter and this will in turn turn more inert material into more combustible... pretty much until someone break this reaction or until there's not much more left of your house than the concrete foundation.
So there are basically 3 ways to put out a fire, wich is taking out anyone of the 3 part of the triangle.
In your case smokeless powder does contain it's own oxidizing agent and it's own combustible so the only thing you can use is the heat element.
So in case of a smokeless powder what you need to do is reduce the heat.
Now about the extinguiser:
They are taylor to the type of fire you can fight, but are not created equals...
They are classified A, B, C, D and K for the type of fire they can effectively fight, let's take out D and K wich are respectively for metal fire (D) and kitchen/fat/oil fire (K)
A: Ordinary solid fire (aka paper, wood, etc..)
B: flamable liquid and gas
C: electrical fire (meaning A, but you can't use water since it would result in electrocution...)
Now most household extinguisher are rated ABC, meaning you can effectively use them against any fire of the A, B or C type. That being said they are usually made of Dry chemical meaning that the basic principle is that on a molecular level it separate the fuel from the oxygen therefore breaking the fire triangle (combustible-Oxidizing agent aka oxygen-heat). Now this is your all-purpose extinguisher, it can do pretty much anything but is definatly better for some than others. In this case the dry chemical is usally better for B and C fire than for A, but it will still fight A fire quite effectivelly.
CO2 extinguisher won't work since the principle is to deprive the fire from oxidizing agent (oxygen) by replacing normal air with CO2 this mean that it won't work in well ventilated area (hello foam extinguisher) and that it certainly won't work with smokeless powder wich is by definition a mix of Combustible and oxidizing agent so depriving it of oxygen is not gonna do anygthing (although it MIGHT help put out subsequent fire, but still non conclusive...)
Water extinguisher (A) is just water with presure, it will trow water on the fire and thus reduce it's temperature. Water extinguisher are also quite easy to operate and recharge. Usually recharging it is just a matter of filling it with water and putting pressure in it with a compressor.
So my (uneducated guess) would be an ABC because it's handy in all sorts of situation and mostly a Water extinguiser, beause althought I never fought a smokless powder fire, it appers to me that heat is the weakest link in this fire and thus water would be the most appropriate response.
Then again I am no specialist.
Etienne