Hi,
You can add the weight by default by modifying the values in the "flightmodel.cfg" (Note you will have to do this after every update).
Step1: make a backup of your flightmodel.cfg (example copy it to flightmodel.org
Step2: open the file in notepad
Step3: search for the section called "[WEIGHT_AND_BALANCE]"
Example from the TBM (the first value in every line is the weight, bold below, so here you put in the weight of the weapons):
station_load.0 = 170, -15.54, -1.5, 1.5, TT:MENU.PAYLOAD.PILOT, 1 ;15.2
station_load.1 = 170, -15.54, 1.5, 1.5, TT:MENU.PAYLOAD.COPILOT, 2 ;15.2
station_load.2 = 0, -19.325, -1.5, 1.5, TT:MENU.PAYLOAD.FRONT_PAX_LEFT, 4 ;19
station_load.3 = 0, -19.325, 1.5, 1.5, TT:MENU.PAYLOAD.FRONT_PAX_RIGHT, 4 ;19
station_load.4 = 0, -22.788, -1.5, 1.5, TT:MENU.PAYLOAD.REAR_PAX_LEFT, 5 ;23
station_load.5 = 0, -22.788, 1.5, 1.5, TT:MENU.PAYLOAD.REAR_PAX_RIGHT, 5 ;23
station_load.6 = 0, -25.72, 0, 1.5, TT:MENU.PAYLOAD.REAR_BAGGAGE, 6
station_load.7 = 0, -11.4, -1.5, 0.6, TT:MENU.PAYLOAD.FORWARD_BAGGAGE, 6
Hope it helps,
Marcel