I always carry a pocket kit if I'm more than a mile or two from people. I'm clumsy enough to fall and break my leg so a fire/water/painkiller kit that'll keep me alive overnight is a good idea.
If I was adding one to a rifle, I'd put most of it on the sling or warpped around the stock. Most of that can be left even when using your rifle and anything bigger, like a poncho, can be shoved in your pocket.
My must-have items are:
5-10 waterproof, longburn matches (for wet, windy weather)
a ferro stick to use as a flint, one with a key attached
tinder, usually cotton balls packed into spare spaces
water purifier tablets
a couple of heavy duty condoms as water holders
some codiene tablets, tiny and good for pain & diarrhea
compass
little knife, like a small opinel
I usually as more, like thread, fishing kit, reflecting mirror etc. ~The painkiller, even simple ibuprofen, is something I consider vital. The above items fit easily in a pocket or pouch attached to the rifle. I'd wrap a small kit like that in tough tinfoil which can be used a cup or even a basic cooking pot.
I'd also add more ammo, either on the stocvk or on a sling pack (or both).
Edit: I know some people scoff at a kit like this but I've benefitted once when I was hiking in a storm and had to ditch my pack during a mudslide. It made the night much more comfortable.
