It’d depend on what rifle and what kind of era (WW1,WW2, Vietnam, Modern). Same with the type of sling used (M1917 leather sling, Kerr No-buckl, single-point, 2-point, 3-point)
It certainly depends on the weapon itself and its sling; but also, what the soldier is actually doing!
A couple of decades ago there was an article re figure modelling in, I think, Airfix magazine; the author proposed that in order to model a suitably posed figure one must apply the following criteria:
What?
Where?
When?
Why?
There may have been another; if so, I’ve forgotten, but I’ve found this quite useful over the years as a sort of check.