User blog comment:Master Moridin/A (Potentially Unnecessary) Explanation for Inconsistent Ammo Count/@comment-26397825-20151107053015/@comment-4010415-20151107113300

If Weiss uses Dust crystals instead of powder, that would mean she would need crystals that are specially cut into cylindrical shapes that are exactly the right size to fit into Myrtenaster's revolver slots. I highly doubt that's the case.

On top of that, we saw in The Shining Beacon that she packed suitcase of vials of powdered Dust.

Myrtenaster probably has some kind of thing going on where she inserts the vial with the cork still in it, and something in the revolver chamber stabs through the cork so that the powder can be extracted.

As for how it looks when it's in the revolving chamber, chalk that up to someone just not feeling like going into the extra detail of modeling and texturing it to actually look like that's a literal glass vial of powder.