I think in hindsight this makes more sense. In this arc, they made a point to explain the pre-conditions of having to have been transformed and also fighting to the end (which means actually the magical girls can run instead of lose, and they're not prosecuted). That's likely a mcguffin.
It could end up somewhere in-between of course. Maybe the gun didn't work at all (as implied, a lot of that evil prof's inventions don't), but Yuuri just forces it as evidence to show she wasn't transformed because nobody can know for sure that was the case unless they outright shoot someone again.
The problem with this is that it depends on the legal definition of being "transformed".
Although Yahiro isn't wearing her magical outfit, we still see her using her powers to fight Dark Harassment. Since it hasn't been stated that magical girls can use their powers without transforming, we have to assume that they need to transform before being able to use their powers.
Since we do see Yahiro using her powers again Dark Harassment and one the prosecution's arguments is that Yahiro fought without using her magical clothing: it's heavily implied that Yahiro was considered "transformed" at the time. Otherwise, why is it that Yuuri didn't make an argument about Yahiro not being transformed at the time? Instead, she decided to pull a wild card on the slim chance it might work.
Of course, the trial is not over yet, so Yuuri could still make that argument. But it's a huge oversight for a defense lawyer not to make that argument before the gavel was mere seconds from being dropped.
