ok so a shotgun shell weighs .1 right. and 30 of them weighs 3kg in the main inventory. and a box of 30 actually weighs .3. a fanny pack weighs 1kg, and contains 1kg and according to the devs will have some increased or decreased weight reduction value that is certainly not 100% and also imposes a movement penalty for wearing it. unpacked from a box you can hold 10 shotgun shells and it will still weigh 1kg for the pack and 1x kg for the capacity where x is the decimal weight reduction coefficient.

let's recap so far. for the sake of demonstration let's say that x equals .2

• fanny pack 1kg
• 10 shells 1 kg
• fanny pack full of shells 1.8 kg
• box of 30 shells .3kg
• 30 shells 3kg
• 30 shells in a good backpack .45kh
• box of 30 shells in a good backpack .045kg

and then let's draw some conclusions

• for the same weight as a fanny pack you can carry 8 more shells
• keeping shells boxed allows 10x as many shells for the same weight
• keeping shells boxed in a good backpack allows 66x as many shells for the same weight
• accessing bags creates a delay even when automatic, which is at most equal to that of unboxing shells and at least more than having loose shells in the main inventory.
• bags create a movement penalty and a player is almost certain to at least have a main bag penalty plus a weight penalty and possible exertion penalty.

so, it seems to me that there's just better strategies than putting your shotgun shells in a fanny pack. the very first strictly superior strategy is just having loose shells in your main inventory because it's faster and weighs less. in the event that you need more than 30 shells the reload time is already longer than the access time could possibly be and if you can't find your shells in your backpack i suggest keeping things sorted by name and looking under B for box of shells or S for shotgun shells. if you are using more than 30 shells you're really either driving around in a car shooting from the window, behind some cheese barrier grinding skill, or in a very bad situation where you probably need all the exertion you can use and need to keep weight to a minimum. in which case honestly i'd drop the shotgun and leg it. or you're going out with a bang in which case none of it matters.

so the only valid argument i can see for a fanny pack is being an absolute Chad who wears it for the RP value and because you need to play with a handicap to feel alive. in which case i'm 110% in support.