Things You'll Need
Instructions
Download the Portal gun mod. The mod is available for free from a variety of file hosting sites, including ModDB, FilePlanet and GameBanana. Once downloaded, the file will be in a compressed folder.
Extract the files from the compressed folder with the file extraction program of your choice, such as WinZip, ExtractNow or WinRAR. To extract the files, right-click the compressed folder, click "Extract..." and select a directory to extract the files. Any directory will do for now.
Replace the "shotgspa.txd" and "shotgspa.dff" files in your "Grand Theft Auto: San Andreas" directory with the "shotgspa.txd" and "shotgspa.dff" files from the mod you downloaded. The files can be found in the "Models" folder. The next time you start the game, your shotgun will be replaced with the gun from "Portal."