Nice catch. Stuff like that is why I was really grateful for my alpha testers back when I was working on my roguelike - they would manage to discover things I hadn't even thought about planning for.
It does look like there's some extra room to the left there, and maybe the whole thing could be shifted trivially - but I would imagine having that much ammo is an edge case, and it might be better handled that way as laptop recommends; that is, sanitize the output so that it's always 3 characters or less.
Either one is equally viable as a solution IMO, so it probably comes down to which is simpler to implement.