NEXTGEN, when the gun can't find it's muzzleflash it'll leave the flash object untextured. making it appear as a white square. THe same goes for the smoke caused by the gun.
If I were you, I'd re-install the pack.
open one of the gun's gunspec files :
you can find it in - files\gamecore\guns\model pack 9\GUNNAMEHERE\gunspec.txt
(you need to replace GUNNAMEHERE with the name of one of the guns.
back up the gunspec, and the replace the following fields with the supplied text:
muzzleflash=1
smoke=1
if you still get white squares, then it's not a problem with the pack, but your FPSC version.
if the white squares are gone, then it's a problem with the gunspec not specifying an existing flash/smoke image