From a discussion in the 1.12 experimental discussion on the forum.
The inventory system needs a bit of an overhaul. You simply should not be able to put long rifles in backpacks or axes/pickaxes, AKS74U and similar large items in clothing.
Ideally I would want inventory space in each item divided into compartments roughly corresponding to number of pockets. So say a wool coat would have its 30 slots (5x6) split into four compartments, two 2x3 and two 3x3. However that would make inventory management an annoying game of tetris and would not work with the system on consoles.
So, please consider tagging bulky items with a "won't go into clothing" flag, it would be simpler and would work with the console inventory system. Long rifles should simply be blocked from going into anything but storage containers (crates, barrels, sea chests, tents). Possibly consider adding attachment slots as on some modded backpacks - then people can see you're carrying a rifle strapped to your backpack.
If folding buttstocks were made to actually fold (maybe that is planned?) that could make those rifles smaller to fit into some bags.
Also, in my opinion you should not be able to bind items carried in your backpack to the hotbar at all, there is an annoying "I-look-harmless-but-have-three-assault-rifles-in-my-backpack-ready-to-pop-in-a-second" meta that is unrealistic and unbalanced. The player should be forced to make choices on what to carry and where to put it.