Does WB have rules, or a description of target scope for WB, that could prevent this kind of factual inquiries on bullets?
Is it off topic for being about factual stuff?
No, WB.SE supports this sort of question. When Goodies says "0% science fiction", this is irrelevant because real world questions are officially on topic as long as they provide enough context to let users know why you are asking the question, can not be answered by a simple Google search, and includes what requirement your answer will be judged by.
When he says "0% story context" he is actually misunderstanding requirement #1 because the official guidelines are not for STORY context at all. Story context is plot which IS off topic here. By "context", the guidelines simply mean that the question includes enough information so that users know "what kind of answer you want."
This question specifies that they want a bullet that can pernitrate a certain amount of armor at a given range which satisfies why they are asking the question. It includes research notes that proves the OP tried to answer the question on his own, but could not. It also states that he is looking for the best option between the two for an anti-armor round satisfying the 3rd point.
So, as for the question if this is on topic for being about real world stuff, then answer is that yes it is.
Is it off topic for being about Bullets?
Bullets (or more specifically, the terminal ballistic properties of bullets) are a mix of technology and physics which are both specifically listed as on topic for this site. So it passes this test as well.
Does this question demonstrate a World Building Purpose?
Sphennings posted this question in his answer but does not cite it. Nowhere in the official rules is this actually necessary. You do not need to say "I am writing a story" in your question. We have rules on this site about what is NOT world building like questions about an established franchise or questions about plot and character development, but there is no rule that says you explicitly have to say what you are working on. That said, even if you were apply this unofficial requirement to the Referenced Post, the OP of this question specifically states "I am working on a sci-fi thing" completely invalidating this argument in this case for a reason to close a question. No, he does not say what his sci-fi thing is about, but we do not need to know any of that to be able to answer the question as asked or to know that he is in fact asking this question in a World Building capacity.
Should the cited question have been closed anyway?
Possibly... while the question is fully within the official scope of WB.SE, there is always the possible issue that a question needs more needs details or clarity. Not everyone has the same ideas about what "needs details or clarity" means because not all possible answers need more questions asked. For example, this question could easily be answered from the perspective that the only factor the OP cares about is penetrating the example armor, but there may be other caveats to consider. Knowing how important things like cost, reliability, versatility, ammo capacity, accuracy, carry weight, etc. are to the OP would help set clearer goals for a best possible answer.
That said, if you are going to VtC a question for lack of clarity, you should always explain why. While this is not an official rule, it is extremely unkind to do because it does not let the OP know what they need to do differently to get the question reopened later.