155s weigh about 100 lbs each. I know this from my job.
Okay, than picture a box 6' X 6' X 12' (my measurements) configured with 155mm barrels and each barrel loaded with multiple rounds of 155's. I don't think 155's have a shell, they use sack charges, correct. Behind each shell would be the charge, and only the rounds entered into the fire control system would fire.
They have pictures of the various configurations on the website.