Guns and the rest of the weapons were banned in the Church. However, that only applied to what were traditionally considered weapons.
The hidden weapon aspect was still not developed beyond simple daggers, poisons, and curses. The mechanical aspect was never considered, as it was never considered whether the person who went inside could build a weapon using parts they had spread throughout their body. And that's what Sheva did.
This whole assassination plan of hers depended on this single moment, the time when the opponent would put down her guard over her surefire victory.
It's why she hid parts of a very special one-shot pistol in her coat.
Originally with the eyes on her as well as Heroth, it would have been much harder for her to assemble the weapon, but once she was thrown down by the Knights earlier while they focused on Heroth. She did her job and built the gun in a matter of seconds and hid it in the spring-loaded holder in her coat.
