I was also thinking certain items should be able to be carried without any volume use-up with a special item. Maybe the holster would remove volume from one pistol instead of just adding a bit of volume like it does now. And a strap could be an attachment to rifles/SMGs that make it so they dont have any volume-usage.
Also, how about a 'camping backpack’
Same as a normal backpack, but if you have it, volume on one tent and one rollmat arent counted.