Part of that had to do with her psychological resilience. And maybe another part had to do with how much was on the line as a female recruit. But she also might have been able to carry the gun the rest of the way because she was a woman. Supposedly, she even did it with a smile. When the man she relieved wrote his evaluation of her (all Ranger peers have to write such evaluations), he said he was particularly struck by how enthusiastic she was in that moment. There he was, completely broken, and she was practically chipper.
