*facepalm* I never realized she used .223 pistols. They're described as sawed-off .22 rifles, so of course I assumed .22LR. Doesn't help that I didn't actually play any Fallout games until AFTER reading FoE. Reggie just reached Best Griffon status, because That Gun.
I don't know about real life, but in Fallout, 5mm is basically full-auto-only armor-piercing round. Nothing in the series fires 5mm semi-auto, because the damage would be stupidly low. In New Vegas particularly, it's only used in Miniguns and Assault Carbines.
I just can't imagine a pistol in the Fallout or FoE universes chambered for 5mm.