While he wasn't an outstanding marksman in the service he was a Marine and there's no Marine that's an untrained shooter.
I would also observe that having seen the setting in person I've never seen a video or a photograph that accurately represents how close in everything is. As an average marksman my first thought was "with a round chambered I could do that". True I've never fired a Mannlicher-Carcano but I just don't see it being as hard as some have portrayed it.
The Mannlicher-Carcano was such a fine weapon that, at the time, it could be purchased in a Burlington, VT Woolworth’s for under $20.