What would Rushuna have done if she wouldn't have been so ... uhm ... well-endowed? Would she have worn a pouch in front her chest? Or would she have put the bullets somewhere else? This troper shudders at the mere thought of the latter option, since human anatomy really doesn't leave that many other possibilities.
I don't think it's really her boobs that are part of her strategy. A warm hug would carry the same effect in a critical moment, busty or flat.
Put the bullets somewhere else? Like ... on her hips? Sure! She'd carry the pouch high on her left hip, pop her butt to one side, and 6 bullets would come flying up, all oriented in the same direction. That *was* what you were thinking, right?
Besides, how do the bullets stay in place anyway? It doesn't look like Rushuna is wearing a bra or tight top. And won't all that metal pressing against her skin seriously start hurting after a while? It sure gives a whole a whole new meaning to the phrase "boobs of steel".
She probably does have a pouch or something, sewn in the front of her dress. Then again, this anime is not the most serious ever made...