I notice that several of the questions about weapon design are solely about the weapon. They appear to make no effort to integrate the weapon into a world.
What do others think?
I would say what @MonicaCelio usually writes (occasionally to me): if you think something is off-topic, vote to close it. People may or may not follow your lead, but you should help maintain a consistency throughout the site. Recent cases have shown that sometimes off-topic sometimes evade the usual voters/mods. If you are unsure about a certain question, do ask on meta about it.
Some appear to be borderline according to the help page.
For example, questions are welcome that are about:
Creation of elements of a world (languages, species, buildings, etc.)
Weapons are elements of the world but the questions should also include how the elements helps defining/altering the world. If the sole focus of the question is only about the weapon, then I believe it's not a worldbuilding question. If your not sure but you think it's off-topic, you can flag it. If enough high reputation users agrees with you, it's probably off-topic.
It's possible that some of these weapons are already integrated into the world, and the askers are just asking how to plausibly account for them. For instance, my question about a sound gun is based around a weapon I wanted to have in my world, and I just wanted to know how it would work, or if it was plausible.
Come to think of it, I assume that's what the reality-check tag is for: questions not specifically about worldbuilding, but about building individual aspects of that world that you'd like to have, but aren't sure are possible. In many cases, I don't think it's necessary to flesh out why you're using the weapon or what other aspects of the world exist, so long as you provide the materials available for the construction of said weapon.
Allow me to point out this question, where the accepted answer pretty much states that you don't actually have to have a world in mind when asking a question. As long as that question is on-topic. There might be a chicken-and-egg scenario here.
I admit it's sort of a grey area in many cases, though. I get the feeling we're having way too many 'is this worldbuilding?' questions here on meta, and the answer usually comes down to what you as an individual (with close votes) decide.
My own preference and the reason I don't tend to vote-to-close except in blatant cases is that I would prefer a relaxation of the rules. I suppose, on reflection, what I'm really doing with this specific question is crying "Unfair!" when I am treated strictly and others aren't.
That doesn't mean I want to bring others down with me. Despite the title, I don't object to such questions. I actually think they are very useful.