Originally posted by jawats I think of "shoulder" bag as a messenger bag...
but, I think you're right to point out that backpacks also rest on the shoulders, and are therefore "shoulder bags."
No, the terms define the purpose of the bag with backpacks the straps go over both shoulder as designed, with the sack or bag covering across the back and distribute the weight.
With a shoulder bag, the straps are generally shorter, designed to carry light and non bulky items, but if the straps are long enough to sling cross body to both distribute weight and secure the bag so the hands are free, it's a sling bag. For the kit you are describing, you might find the weight inconvenient on just one shoulder, and shoulder bags often slip unless secured by one hand. For those reasons, I've often chosen a sling bag, though I do use a holster as well.