Besides that gas situation, she had to be careful not to waste ammo, there's also the issue of them being faster and if she got caught with the back turned it's game over.It would probably be helpful if they used their guns when the monsters are at the end of the long hallway instead of waiting until they're right in front of them.
Gamon submitted the manga as a complete work, it's just being released in installments.The action is flowing pretty well now. I'm curious how long the author can keep up the pace