It's a dumb way to carry a rifle. You have no control if you fall.
I carry mine (in Africa or elsewhere)slung over my shoulder. If game is near, I carry it in both hands.
DG rifles are not that heavy. 10 pounds or so.
So why do they carry them so? In the old days, they had gunbearers. The gunbearer walked in front, rifle butt pointing backwards, so you could easily grab it from him and shoot. Nowadays, no gunbearers but the habit lingers on.
