If your textures are already in TGA format skip this.
Open each texture.
Go to File>Save as and save as a Targa (.TGA) file.
Put these Targa files in a folder called lr300.
If you are animating a different weapon make the folder name meaningful.
Make a .bat (text) file with the following code:
cd .\counter-strike source\cstrike\materials\models\weapons\v_models\
The batch file will not work if these folders are not created. If this is the case, remove the second and third lines of code from the batch file. Now with the new window open, go up one folder. Create more folders in the following hierarchy: *username*(folder that you are at)\counter-strike source\cstrike\materials\models\weapons\v_models\ If you did this correctly the batch file with all four lines of code will work.
Execute the batch file. Now you will see your materials folder for first person view models. Keep this folder open for later.
The camera in XSI will be incorrect now. To fix this, unhide the camera. Move it to half way of each axis done on the origin. (e.g. If you put $origin 0 3 0, Translate the camera on the y axis 1.5 units. Set a key for this new position (K).