This may not be the answer you’re looking for but when I’m carrying a barrel heavy rifle I just strap it to my pack.
I’ve also had success with the claw rifle sling installed on a rail with a QD mounted as far forward as I can get it.
McMillan makes my favorite rail, maybe mounting one far forward could help?
I use the biathlon sling mentioned above for when I am hunting without a pack, I do also own and use the Kifaru Gun Bearer for use with my pack. I do use the Kifaru more than anything. The long barrel does stick out in front of my face a little but it also makes for a steady rest for my binoculars when I stop and glass for a few minutes.
The solution to carrying a heavy rifle for long distances. The padded shoulder straps distribute the weapon's load to both shoulders, instead of one, resulting in less shooter fatigue after long periods of travel. Having the load centered on the back aids in maintaining better balance when using...