I noticed the following discrepancies when using the in-game E6B weight display:
1) There is no weight increase when loading RS-132 rockets on the Il-2.
2) The MK108 cannon (FW190A-8, Bf109G-14, ...) weighs 194 lbs (88 kg) in the game (without ammo). All sources I found state 58-60 kg.
