In reply to Robert Lyons: I've used an AK*37* for carrying around 17kg of rack, rope and bivvy gear, no problems for me, but then I dont mind teh lack of back padding, my larger sac is a POD expodition, but I always pack too much if I'm taking that, so its releagted to week plus trips
AK 47 should suit well? Easiest way of lightening up a rucksack is not to fill it!
chiz