Passage: Fixed armament of the Q-5 was reduced to two Type 23-1 23 mm cannon with 100 rounds per gun, mounted in the wing roots. Two pylons under each wing and two pairs of tandem pylons under the engines were provided in addition to the weapons bay. A total of 1,000 kg (2,205 lb) of ordnance could be carried internally, with an additional 1,000 kg externally. On many aircraft the weapons bay is now used primarily for an auxiliary fuel tank.

Question: In a lot of aircrafts, what is stored in areas where weapons used to be?
an auxiliary fuel tank