ZUMWALT PRAIRIE, Ore. — Aiming a rifle loaded with a copper bullet rather than the standard type made of lead, Chelsea Cassens fired at an elk from 70 yards away, hitting it squarely behind its shoulder. To avoid spooking the animal if it was only injured, Ms. Cassens waited several minutes before approaching as her father needled her skeptically, suggesting her newfangled ammunition might not have immediately killed it.

Moments later, Ms. Cassens, her father, Ed Hughes, and the three others in their hunting party descended on the fallen 450-pound beast, carved it open, inspected the internal damage, and found the spent bullet.

“Will you look at that!” Mr. Hughes said, pleasantly surprised. The copper bullet had expanded on impact, as it was designed to do, opening a gaping hole in the elk’s lungs and killing it almost instantly.