When installed correctly the old military style sling is ideal for shooting off hand, kneeling or sitting. Rather than trying to explain it, do a search on the net which will show the proper installation and use of it.
In my over 50 years of hunting I've only used my sling once to help steady my rifle. All the other times the animal was close enough that it wasn't necessary, I was either sitting or kneeling, or I had time to use a tree, back pack or some other form of rest.