5 million rounds
I heard a terrorist boast once that they had 5 million rounds of M-16 ammo .223 or 5.56 x 45mm that they had somehow pilfered from the Israeli army. I searched long and hard and came up with a single source that says 1000 rounds more or less weighs 40 lbs. I like metric, That comes out to around 18 kg.  18 (kg per case) x 5000 (cases) = 90 tons. A large Tractor Trailer carries about 80 tons. (I cannot find a page to support that statement.) I suppose that is a possibility.