Assuming 147 grain bullets, a case weight averaging about 60 grains per case, and let's say 4ish grains of powder per round:
(147 + 60 + 4) grains/round * 71 rounds/drum * 9 drums = 134,829 grains
134,829 grains / 7000 grains/lb = 19.26 lbs of rounds.
Plus the weight of the magazines and the...