Personally, I think the magic railgun is a long shot (pun intended) and don't care much for it.
I respect your opinion.
I personally think Magic Guns Are Cool, and that we are uniquely situated to eventually innovate a really neat Magic Gun, but I also acknowledge that this is a cooperative forum game, and that not everyone is as interested in the subject as I am.
If Divided Loyalties taught me nothing else, people sometimes just aren't interested in even looking at the options for Fancy Warhammer Fantasy Guns, let alone buying them, and nothing I do will change that.
I am not sold on Casandora having a railgun. Or it being notably better than hitting people with lightning bolts even if she did.
I'm not against looking into it at some point, but basing her whole build around it? No way.
As I understand the IRL mechanics involved, a personal scale railgun within the capacity I am imagining in my head would have a longer range than a lightning bolt, would not be dispelable or counterspellable, would ignore Anti-Magic defenses, and
might have increased penetration compared to a lightning bolt, either in terms of armor piercing, multi-target in a line striking, or both.
Hypothetically with a massive amount of budget and a whole lot of research, we could then create a warmachine sized rail/coilgun, at which point it should be able to reasonably attempt to snipe dragons out of the sky or shoot holes into Karak Walls, but I would expect us to have the resources and research of a Lord Magister by the point in time we begin to even start to get together the funding to properly investigate upscaling the idea from personal scale usage.
I can't comment on whether or not such an investment would constitute "basing her entire build around it", because honestly this all sounds like stuff that, on completion and actualization, could be worth earning her Magistership or solidly mark her record positively in regards to Lord Magistership, in terms of innovative research at the very least, so I cannot say that if we sufficiently investigated and researched electromagnetic forces to the degree that creating an Electromagnetic rail launcher wouldn't be an immediate and immense risk to Cass's life, that it would not constitute a major aspect of her build, especially in context of having an Elementalist Master teaching us.
So I can't make any sort of argument or discussion on your second point beyond "That is a fair opinion to have. Good day."