I suggested that it may have been worth trying to remove all the guns, and fair over the turrets.
Bomber Command's Operational Research Section calculated that in 99% of combats the mid-upper turret needed no more than 1,000 rounds, while for rear turrets the figure was 2,000 - the total stowage capacity was for 18,000 rounds (around 1,000 lbs weight). They recommended that rear turrets should have 3,000 rounds, rather than 10,000, but it's not clear whether this was implemented.
The ABC Lancasters would have been flying in the bomber stream and thus over the target, so it would seem to be a waste not to drop at least some bombs.