AuraTwilight
The Sorcerer of Smiles
- Location
- The Silver City
We know that they can't survive a direct hit from a .357 Magnum/.44 Magnum/.50 Action Express handgun round (depending on the specific caliber of the Desert Eagle that Homura stole from those yakuza). That means that it's weaker than a Level II or Level IIIA ballistic vest. So, not really very tough at all.
I honestly half-suspect that Homura might subconsciously enchant her guns to count as magical attacks, so knowing the fragility of Soul Gems is something I'd prefer to ask Kyubey.