How can I tell if these short-falling shots are a bug?

Start with repeatable tests rather than a few surprising salvos. Keep the ship, range, aim point, and firing conditions the same, then record where each shell lands. A clear pattern across repeated attempts is more useful than a claim that shots seemed unusually short.

The display alone may not tell the whole story, so document exactly what it shows alongside the results. That gives other players something concrete to check without assuming the cause in advance.

It could be a bug, but the complaint by itself doesn’t establish that. I’d look for a consistent gap between the indicated dispersion and actual impact points, and for enough detail that someone else can reproduce the test. If the reports don’t include those details, it’s hard to tell whether they’re describing the same issue.