guns can’t have negative damage values anymore. Also, copy from 10mm in this case would copy the 10mm item from mainline and then override its data according to the values given. It’s standard practice to futureproof stuff
edit: well, not specifically from mainline, but the 10mm item passed down from mainline, subject to any other mods that copy from 10mm and modifiy it. Last hands on it gets the win.