I think the only reason some people really have this issue (and do not mix it up with their own human error) is a bug in loadouts saving. Loadouts are saved in the user.ini file in Docs/1C folder. Section for each aircraft looks like this:
Code:
[Aircraft.SpitfireMkIa]
conv _Gun03 Gun.Browning303MkII 183 600
conv _Gun06 Gun.Browning303MkII 183 600
conv _Gun00 Gun.Browning303MkII 183 600
conv _Gun01 Gun.Browning303MkII 183 600
conv _Gun07 Gun.Browning303MkII 183 600
conv _Gun02 Gun.Browning303MkII 183 600
conv _Gun05 Gun.Browning303MkII 183 600
conv _Gun04 Gun.Browning303MkII 183 600
belt _Gun03 Gun.Browning303MkII MainBelt 11 9 9 9 9
belt _Gun06 Gun.Browning303MkII MainBelt 9 9 11 9 9
belt _Gun00 Gun.Browning303MkII MainBelt 10 11 9 9 11 9 9 12 Residual 20 ResidueBelt 8
belt _Gun01 Gun.Browning303MkII MainBelt 11 9 9 9 9
belt _Gun07 Gun.Browning303MkII MainBelt 10 11 9 9 11 9 9 12 Residual 20 ResidueBelt 8
belt _Gun02 Gun.Browning303MkII MainBelt 9 9 11 9 9
belt _Gun05 Gun.Browning303MkII MainBelt 11 9 9 9 9
belt _Gun04 Gun.Browning303MkII MainBelt 9 9 9 9 11
beltPreset AM1 _Gun03 Gun.Browning303MkII MainBelt 11 9 9 9 9
beltPreset AM1 _Gun06 Gun.Browning303MkII MainBelt 9 9 11 9 9
beltPreset AM1 _Gun00 Gun.Browning303MkII MainBelt 10 11 9 9 11 9 9 12 Residual 20 ResidueBelt 8
beltPreset AM1 _Gun01 Gun.Browning303MkII MainBelt 11 9 9 9 9
beltPreset AM1 _Gun07 Gun.Browning303MkII MainBelt 10 11 9 9 11 9 9 12 Residual 20 ResidueBelt 8
beltPreset AM1 _Gun02 Gun.Browning303MkII MainBelt 9 9 11 9 9
beltPreset AM1 _Gun05 Gun.Browning303MkII MainBelt 11 9 9 9 9
beltPreset AM1 _Gun04 Gun.Browning303MkII MainBelt 9 9 9 9 11
...
loadout Default SpitfireMkIa_WingGuns Default
loadout MyLoadout SpitfireMkIa_WingGuns AM1
1st part is convergence settings
2nd part is default weapons loadouts
3rd part is my custom weapons loadout named AM1.
EDIT:
4th part is the
aircraft loadouts names (selected on the page where fuel is set) and corresponding Weapons loadouts (selected on weapons page).
Just check that you do not have any inconsistency or strange records for any plane. E.g. if I name a custom loadout using localized Russian characters I can get 2 sections for default settings one of which is named in Russian which is wrong. Also I noticed ammo numbers not separated by spaces, etc and corrected them all.
You can delete the file and let the game create a new one and then edit one British plane and copy-past to other British planes, etc.
To be on a safe side you can set your default loadout the same as your preferred custom loadout.
Check that all names in the 4th part are correct.
Loadout changes may not influence SP missions. Also when you edit loadouts via GUI you have to hit Save (or save as, can't remember now) in both screens: weapons settings and aircraft settings. Otherwise something may not save. If Save does not work, try save as. Use international characters only for naming. If you save everything correctly and keep your settings file clean everything works.