Why can't we use boron filament as railgun armatures?

Boron and silica aren't electrically conductive, at least to the point where the game will let you use them in railguns even if they weren't fibrous. Most other fibres are comparable, being made of polymers. However something like metallic microlattice (which gets classed under fibres due to being porous) might have some application like producing very large shots to increase missile hit chances.

Would a fibrous armature behave like chain-shot? Fancy that for some radiator shredding.

Isn't the problem with fibrous material the lack of contiguity? Pretty much the only reason I can think of would be the fact that electricity could potentially explosively jump through the amerature, detonating it in the barrel. Especially in the case of capacitor based EM guns.

The concern to electrical continuity is expressed in the resistance, for materials, where the microstructure is significantly smaller than the projectile size. Similarily, heating from resistance would occur uniformly, as it would in metal projectiles. The fact that most fibrous/porous materials have a high resistance is no reason to exclude them from potential use.

As for the ability to hold up to acceleration, the materials ability to withstand high g accelerations is ultimately given by its ultimate tensile strength. This property is already in the game, so I see no problem there.

Don't forget that all of the materials in game have a microstructure, even the alloys and pure metals, as none are expresly monocrystalin. The difference in this case is mostly one of scale. As long as the scale of the projectile is sufficiently larger than the scale of a materials microstucture, it can be treated as we treat projectiles now.

Please gieve context to that image, I can't tell what it is, or properly read the scale. Also note that I stated that the structure of the material needs to be sufficiently small compared to the projectile itself. The image does not seem to show a sub structure that is small compared to the overall size.

As for "the projectile will crush the armature": first off, few people use projectiles infront of the armature in CoaDE. In most cases the armature is the projectile. As for the "crushability" of a material: I'll say it again, the game uses material constants to define how a material reacts under stress. These will tell you how a material reacts to a compressive force provided the scale of the compression is sufficiently larger than the micro structure of the material. This is true for polycrystaline materials as well: the in game constants are only true, given forces applied on a large enough scale so that many crystal boundries are within the effective volume.

The image shows a 3d model; the original armature they were modeling was a made of solid metal. On the diagram max temperature is three times higher than min temperature. Full article

Nobody argued that you can't simulate porous materials with existing tools. I compared their properties to bulk materials. For example, this article says that electrical properties of porous Fe-Cr-Al alloy are very different from this same bulk material. Moreover, these properties change non-linearly with the size of pores. While at it, also note the pattern of heating of their lattice.

Assuming the game has the same rule of all fibers in how much "gapage" are in between the fibers, then it's entirely feasible for us to use filaments as ameratures.

I still think they'd be suceptable more to melting (or ignition), and generally coming apart around the edges than the typical bulk material.
If the fiber is nanostructured, this should be much less of a problem.

1. Yes, there is a very good reason why armatrure shapes akin to this are chosen. The strakes at the side are to prevent arking as far as I'm aware (see this site). As for the curved design see this thread. This has nothing to do with the material an armature is made of however.

2. In the artical about the electric heater, the current is aplied form a central to an outer conducter, and travels outward from the center to the edge of the foam cylinder. This means the volume the current travels through as it nears the edge of the cylinder is much larger than at the center. For materials that heat up homogunously from resistive heating, heating is dependant on current per volume. This means one would expect significantly less heating as one aproaches the edge of the cylinder, which is exactly the results found in the paper. The high temperatrures at the center of the element have nothing to do with the material, but rather with the design of the heater.