A bit of a thread hijack.
Raksan,
Are you on a Mac or PC? I'm not certain if the directory structure is the same.
The default gunsight for all aircraft is in the "textures" folder and is called "GUNSITE.tga"
Aircraft specific gunsights are located in the "skins/
aircraftname" folders. Not all aircraft are listed here, but you can add them by creating folders using the short version of the aircraft names listed in the pnames.txt file. For the P-40 series these are P40B, P40E, P40F, P40N, P40N5. I don't think the folder names are case sensitive, but could be wrong.
If your default GUNSITE.tga file is missing from the textures folder and there isn't a gunsite.tga file in the skins/
aircraftname folder, then you won't see a gunsight in the cockpit. Alternatively, if the gunsite.tga file was modified (i.e. some of its content deleted, such as the alpha channel) that could also cause the gunsight to be missing in the cockpit.
Hope that helps.
Phenix has a great write up on custom gunsights, as well as some custom gunsights for download here.
http://www.errthum.com/troy/warbirds/gunsites/history.html Now, over the life of the game some of the short names for some aircraft were changed. I haven't tested any of phenix's files, but some may not use the correct folder name.