Peak Design Everyday Sling 5L
Camera shoulder bag
Peak Design Everyday Sling 5L
Camera shoulder bag
Yes, you can. Just adjust the strap and stow it in the openings provided. There are instructions. About the function: it works well for normal walking and possibly cycling. But the camera should not be too heavy.
My EOS 60D (is a bit bigger than the 77D) with Canon EF-S 55-250 (is minimally longer than the Sigma 18-300) it fits vertically just enough that you can still close the bag. Horizontally it has a bit more space and the camera fits in well with the lens mounted. There should be room for a wallet, spare battery, handkerchiefs and possibly another lens, but then the bag is pretty full.