I don't believe the sniper was sitting on anything during the second and third shots, so his "sitting up" after the final shot makes no sense to me.
He was squatting or kneeling during those two shots, imho.
-- MWT
PS Uh oh. I see Scully's still cuttin'-and-pastin' together yet another entertainin' "post" for us here.
He's been workin' on it for twenty minutes, already.... ..............................
Here are a couple of images showing that it wasn't necessary for him to squat or kneel for those two shots.
As you can see, the rifle is a little above the top of the boxes. If he wanted to use the boxes as a support for the rifle he could have just scooted his ass back a little and leaned forward more in order to lower his eye level. I believe that this would be more stable than a squat or kneel. However, in my opinion, doing that would have slowed him down during the interval between the shots. The more likely case would be to hold the rifle above the boxes. I don't have the ability to adjust the arms and legs on the characters. So you have to use your imagination for that.