In the battle to dismantle gun restrictions, raging in America’s courts even as mass shootings become commonplace, one name keeps turning up in the legal briefs and judges’ rulings: William English, Ph.D.

A little-known political economist at Georgetown University, Dr. English conducted a largest-of-its-kind national survey that found gun owners frequently used their weapons for self-defense. That finding has been deployed by gun rights activists to notch legal victories with far-reaching consequences.

He has been cited in a landmark Supreme Court case that invalidated many restrictions on guns, and in scores of lawsuits around the country to overturn limits on assault weapons, high-capacity magazines and the carrying of firearms. His findings were also offered in another Supreme Court case this term, with a decision expected this month.

Dr. English seems at first glance to be an impartial researcher interested in data-driven insights. He has said his “scholarly arc” focuses on good public policy, and his lack of apparent ties to the gun lobby has lent credibility to his work.

But Dr. English’s interest in firearms is more than academic: He has received tens of thousands of dollars as a paid expert for gun rights advocates, and his survey work, which he says was part of a book project, originated as research for a National Rifle Association-backed lawsuit, The New York Times has found.

He has also increasingly drawn scrutiny in some courts over the reliability and integrity of his unpublished survey, which is the core of his research, and his refusal to disclose who paid for it. Other researchers say that the wording of some questions could elicit answers overstating defensive gun use, and that he cherry-picked pro-gun responses.