A three-litre bottle of cider. Most commonly used in Belfast, Ireland.
In Ireland, the old British police and army stations are often called barracks, and the idea was that the Irish used to fill massive bottles up with petrol, light them, and throw them at these British barracks, effectively "busting" the barracks.
So now, a 3l bottle of cidre, due to its size and volume, is now provisionally known as a 'barrack-buster.'
"What did you get?"
"Just two barrack-busters."