Aetherite

Aetherite is a material discovered during the Middle Empire Period, by many considered to be a Golden Age of Imperium of Karadia. As in case of many such discoveries it is hard to say who exactly was the one to achieve them, due to collective nature of research done by Imperium and the fact that it is always slightly (or very much) behind what many hidden sects and research groups achieved.
Aetherite itself is a combustible magical material created from a mix of promethite, aether, gunpowder and dusted red mana crystals. It is supposed to fix the main problem with simple gunpowder - namely the fact that it can be easily ignited at the distance with proper magic. Aetherite is something essentially magical, being a dust that can be partially enchanted, and is filled with magic during the production that makes it resist such attempts.
Instead aetherite creates lower initial velocity than gunpowder, lowering damages it causes. They are actually lower than those of a crossbow, and its bullets are less diverse (due to simply being smaller) than bolts or arrows. On the other hand, they are precise and there are even rarely encountered semi-automatic or even automatic variants (though they are prone to malfunctions and lose even more of their bullet velocity).
It laid foundation to creation of proper rifles and muskets, not to mention first aetherite cannons that revolutionized process of sieging fortress.

Access & Availability

Creating aetherite requires quite advanced technology... at least when we talk about making a proper military-grade one. It can be easily produced even in the field (as long as one has required materials and some knowledge of alchemy), but without proper rationing the materials the bullet velocity will be lowered. Quite substantially. Despite this, it is still much more often that 'proper' aetherite.