Best hands free rifle sling.

Looking for something to carry my rifle on my pack but also be easily accessible. I’ve seen options from Mystery Ranch and FHF gear. Just got a Mystery Ranch Pintler pack, so I was leaning that way but it has mixed reviews. Is anyone using something they really like?

Strapped to the back of the pack really sucks when you have seconds to make a shot, but the sling sliding off my shoulder all day is really annoying.