It’s not as horrible as I also thought at frist! I have become used to it, but there is a trick to it!
I don’t know how new you are to the game, but it seems new players (including myself) often tend to pick up every type of weapon they find, and try to keep a few stacks of ammo for each of them. This leads to an insane amount of stacking, moving around, dropping, picking up, you know how it goes…
You can mostly avoid this by doing the following:
Max out your Increased Carry Capacity skill.
Don’t carry around every type of utility item. Think through what you’re using, and dump the rest!
Carry a maximum of 5 weapons! Sidearm, 2 primary weapons and 2 secondary weapons.
Prioritize room for primary weapons ammo.
I don’t claim to be an expert at this, but here’s a screenshot of my inventory.
I don’t use the sidearm a lot, so I just keep a full stack of ammo in the weapon, maybe 1 extra stack.
I use a shotgun and an assault rifle for combat. I keep around 10 stacks of AR ammo, and around 6 stacks of shotgun shells.
Rocket launcher and sniper rifle are only for occasional uses, so I keep around 4 stacks for each.
So, the trick to avoid the stacking issues is to always after a fight, drop a full stack of ammo into your weapons. When you loot, it will fill up the incomplete stacks in your inventory, until you don’t have room for more. When all your slots are dedicated (and full) the problem solves itself.