WRIGHTSVILLE, Pa. — Standing inside his cluttered garage workshop, Michael Crumling, a 29-year-old gunsmith, proudly showed off an impressive collection of lead bullets he has meticulously crafted from scratch.

Across the country, millions of weapons aficionados like Mr. Crumling reload their own ammunition and cast their own bullets — time-consuming hobbies that have been a part of firearms culture for generations.

But Mr. Crumling has also created something else in his garage that distinguishes him from his peers, a potential solution to a problem that has long vexed creators of 3-D printed guns: a bullet that wouldn’t ruin the plastic firearms.

And yet Mr. Crumling says he has no plans to sell or mass produce the designer round, despite it representing the next step in making printable guns more reliable, and the next front in an ongoing regulatory fight over homemade weaponry.