WP:GUNS importance assessment
As you surmised, the ten percent standard is intended to be interpreted as an order of magnitude, rather than a difference between a disqualified 9 percent and a qualified 11 percent. I am certainly open to alternative suggestions to clarify the criterion; but I disagree about the absence of sources to define an order of magnitude quantification.

The American gun culture includes a large number of writers who emphasize opinions rather than pursuing statistics. As the internet allows anyone to screen for information sources confirming an existing bias, such authors may be well represented by conventional search-engine algorithms. I personally enjoyed articles by Elmer Keith and Jack O'Connor relating divergent first-hand experiences hunting with light or heavy bullets; but I sought out articles by Julian Hatcher to find results of side-by-side scientific method comparisons.

I believe sources are available to quantify the features qualifying as top importance articles. It may be difficult to find quantification in the popular press (including the anti-gun press); but well-researched sources are worth the effort required to find them.

Assessment of importance requires a fundamental understanding of the subject; and the amount of time required will vary inversely with the experience of the editor. I wouldn't encourage you to undertake this assessment task with an unrealistic impression of the knowledge or time required. As a practical matter, however, you may understand the subject well enough to know the answer after reading the article. If you think the subject represented more than ten percent of the firearms or ammunition, it's probably a top importance article; but it's probably not top importance if you have doubts. I doubt your decisions on that basis will be challenged; but it shouldn't be an unreasonable task to find documentation on the few which might be.