To be fair, even if it's later revealed the turret is meant to be non-lethal, this is likely exactly why she didn't load it.
It'd make sense if, despite her history of building destructable things, Scarlet might actually the most weapons safety conscious of the cast. After all, Quentin apparently tested and dealt with a lot of weapons-grade things. She would have not only been able to observe the consequences of any accidents and mishaps, but might have even overheard lectures and upset chew-outs given by those in charge of safety.
