The eberlestock packs were designed for sniper teams.
The principle was that the primary long gun was the main load and most of the weight being carried. Up until we got eberlestock packs, we used drag bags.....which were more just a big soft gun bag with shoulder straps added as an after thought. The drag bags also carried the long gun muzzle up, which put the top/muzzle way above your head and would hit everything you walked under. When you went prone with a drag bag......the barrel was pushing down on the back of your head. Wearing a drag bag made it obvious from a distance that you were a sniper......and a lot of bad guys have a grudge against snipers.
Drag bags also made the gun 0% accessible without removing the bag from your shoulders.
On the eberlestocks, the scabbard was put closest to your back to decrease leverage applied by the weight of the gun against your back.
Some of the long guns we carried were accessible without removing the pack.....and some were not. It depended upon the length and shape of the gun.
Sometimes we carried the precision long gun in the pack and an M4 as a "ready gun" for the walk to the destination.....it is difficult to carry two long guns any other way.
The volume of the other pockets on the pack varies widely by model......but these packs were not designed to carry huge amounts of sustainment gear or to pack meat out from a hunting trip.
The eberlestock scabbard packs are not perfect.....but they were a big improvement in many regards over drag bags.