@sylvacoer If it goes to show, he was probably not taught how to properly use a rifle while moving on a sled (standing still and on a sled are entirely different and it doesn't really make sense why his grandfather wouldn't teach him, unless he died before he could properly teach him, but I'll leave it to my suspension of disbelief), or that he has only hunted small game using what is essentially an 1800's small game shot gun.
In regards to military training, a seasoned and trained soldier should be just as good, if not better, than a seasoned hunter. The game may be different, but the skill sets are always the same, whether it be man, or other animal.
Besides, from context clues, it really does look like his grandpa died before properly teaching him how to shoot things or use a gun even. And also from context clues, he never got better because if you keep practicing the wrong way.
I do admit its pushing his incompetence a little to far considering he had a decent posture with the blunderbuss in chapter 5.