A wonderful take on a critical piece of literature. I love how comprehensive it is, both the contents of the Biv and the contents of the submission. I also like how its circulation and treatment are a reflection of societal values. So we have society changing the Biv, but the one thing I feel this piece lacks is how the Biv influenced the people (other than obvious literacy). Does it actually lead people to better lives? I don't get the sense it actually had a strong impact on leading people out of the Age of Night. Are there Biv radicals of obscure versions and interpretations? I understand it is not a religion by some definitions, but it is by others and that is why I am trying to precieve it through that lense of experience.
Backstory is good, but the mechanics are dubious. If I am reading this right, the bowstrings fire the bolt down a rifled barrel, but I see this as adding more drag to the projectile, lessening its range and penetration value. The standard crossbow and its all steel arbalest brother seldom lacked for penetration power, and their accuracy was indeed less than that of the longbow, but it was a work in progress. The area where the crossbow fell short, was that it had a slow rate of fire. The bowstring and its potential energy had to be drawn back by hand. The arbalest traditionally had a hand crank to draw the string back since a typical soldier didnt have the upper body strength to do it by hand. A conventional crossbow could be secured with a foot loop and the string drawn back by hand. Two bowstrings doubles this, meaning a big mechanical device to draw both back at one time, or drawing the two seperately.
Rather than being revolutionary, I can see the Sigurdian Bowgun being an anomaly in weapon design, intimidating and powerful at short range, but complicated and difficult to make.
You could go with a spring mechanism, with a winch to draw back a hideously strong metal coil... or you could make it a Gauss crossbow, propelling the bolt along the barrel with whatever technobabble strieks your fancy.
Also, I don't think fletching per se has to impart a spin on the arrow, it ensures that the arrow does not tumble.
As it is, the technological solution does not make too much sense.
On a different note - you can really go way out there: what if the bolts are magnetic - and coincidentally, the magic used to enhance the armors of the cavalry improves all the properties of the metal, including attracting magnets, disproportionately so?