Open the .pk3 with ether WinRAR or PakScape. Once you find the skins you wish to edit open the .dds files with Paint.Net or Photoshop. Save as DXT5 with Mipmaps. Once you are staified with your skins use ether WinRAR or PakScape to make a .pk3 file and add it to a folder within the CoD directory or in the main folder. Remember to name it with z's in front so you know it's not an original file. Something like zzz_boltrifles.pk3.

Remember not to overwrite original game files. I highly recommend backing up your Call Of Duty directories just in case you overwrite a file by mistake.