I have one of these ! :shakeshout:
Mine has an open slot pocket one side so I keep a std gents 'quick draw' folding brolly in there. I put my scope across the back & do the flap up over it with the stand outside, so I only have to carry rifle & mat. I attach one of those waterproof rucsack covers to it in case it pours & a square of camping mat foam in it in case its cold & I dont want a cold bum. The main bit takes the jumper, towels, waterproofs, book, notebook, ammo box, sights, lucky-gonk & soft guff, side pockets = bits, tools, oil & more nessesary guff, there is a zip pocket on the flap for pens & bits of 'longer than wide' guff.
then i got one of these

& a pair of these

& chopped out half the weby bits so I can have 30 rnds in their little holes one side & all my little needed guff in th eother side - like magifying glass, spare eagle eye, ear plugs, snap cap, Ikea pencil (yes stolen), spare foresight element if it rains hard, glare tube, mini swiss penknife, mini screwdriver, rubber, etc.
I have one of those ESE mats but it was so heavy so I undid the stitching one side pulled all the foam out & put that white closed cell packing sheet foam in it & its now 1/3 the weight (& i can pull it out to dry). In summmer i just have an elbow mat.
If the rifle is in a pad bag on my shoulder i have a hand free to.........

Quality control of Scottish Ethanol. & RDX/HMX
& my fav chemical is :-) 1,3,7-trimethylxanthine.......... used to kill frogs.... but widely consumed & in vast quantities by the French? Eh?