I agree with unkated, it's a bit too "magick-gimmick" to have the pack reduce the weight of carried items - because if you do, Players will just have their PCs carry more than they should!
A well fitted pack (as opposed to the way school kids in Australia wear them - hanging down on their arse) helps to distribute weight more evenly and comfortably. What a pack should do in game, is reduce the fatigue penalty rather than reduce the weight of items carried. I haven't thought up any specific rules for that because I don't really know the 2013 rules set well enough.
|