The 151/20 round is ~190 grams, the projectile itself is ~100 grams. 50 spent casings (if I understand you right) is about 10lbs. That's if the casings still had the charge in them, which they don't, so let's say roughly 1/2 the weight for argument's sake. That'd be a difference of 5 lbs of empty casings between a 150-round 109 and a 200-round one.
5lbs, where are they? , not to mention for the 30mm