As an ex soldier, I can give you an idea what we were required to have in there at all times:
MOPP gear (chem suit)
1 pr extra BDUs
2 pr socks
1 pr t-shirt
shaving gear
poncho
wet weather top/bottoms or goretex
2 qut canteen
E-tool
flashlight.
Mission essential gear (NODs, demo gear, clatymore, radio gear, as assigned).
After that, you can put in anything you like.
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my adventures