Vorn Equipment has an interesting quick release for their rifle backpacks.
The backpack holds the rifle behind the right shoulder. You simply pull the orange release ring-pull on the left shoulder and it opens the rifle compartment.
They offer an assortment of backpacks of varying sizes starting at $229 for the smallest 7 Liter pack up t $329 for a 42 Liter pack. They also offer these packs in Realtree Camo.
It looks like a great alternative to a rifle sling or you could keep a sling in the pack in case you don’t need to keep the pack on you and you just want to move freely without the pack.