Not to mention I can't imagine how much space a lot of 400 milk bottles, no matter the size, would take up. Say the liter-sized bottles, theoretically in boxes of 12, that would be 33 some-odd boxes, or even if there was 20 to a case that'd still be 20 boxes of empty plastic milk bottles, unless you have a storage unit or something to store them in I can't see it being a feasible solution. The other problem with storage in growlers or smaller carboys is when you wanna have some mead, you're staring down a gallon to a half gallon container that you will likely not finish unless you're at a party populated by viking warriors. A 750ml bottle is a lot more manageable. Kudos for thinking outside the box, but I think the idea is roundly disapproved.