Leave the shotgun at home, use a good GBB with at least three mags, all loaded with minimum 0.30g BBs (reason being, cuts through brush better, faster firing rate, extra range from heavier rounds, but because you hae limited rounds, heavier BBs hit with more impact, so less rounds required to make the target feel the hit and call out. Could take you 10 rounds of 0.20 or 0.25g BBs to hit a guy 40ft away through brush, but you can do the same with one or two 0.30g or 0.36g BBs. Ammo conservation through skill with pistol and 'smart rounds'.)
Next, drop the loaders as mentioned above, keep sniper and pistol ammo (should be the same really) in a ziplock bag inside of a cloth bag, and load by hand. Slower to do, but you are guaranteed to not put any dents, cuts, etc in your ammo that'll make your BB stray off course because of an added fault. Think the first Sniper movie, Tom Berenger using a fingernail file to smooth out the burrs and tips of his match grade ammo. Your ammo should be the same, match grade (Bastards 0.36g) and take care of it).
Next, HSGI Sniper's Waist Pack, consider this, doesn't do much at all for the load out I am recommending, but only works if you drag along an AEG. Still..........