Agreed with Boomerang.
Most weapons are at least 10 lbs, good thing its still free to change. Is the strap paper? If not try painting a leather color on paper and cut out the strap from it when dry. Use acrylic paint so it soaks in. When fastening the top strap point it upward to simulate the weight when slung over the soldiers shoulder. Keep it tight as possible to his back and even have the stock pointing forward slightly under his arm.
For a weapon slung over shoulder during a extended period it would be adjusted longer and over the left shoulder, around the right hip for a right handed person. Weapon pointed up and across the back with the magazine pointed upward.
If leaning on the stump glue the strap on pointing down. Also it could be set on the top of the stump after the soldier removed the snow, with strap hanging down.
Great vignette and always admire your skills Jtracing