Select the replacement model.
Change FOV to 65.
Load weapon, for example: w_models\weapons\w_shotgun_spas.mdl
Use the corresponding "idle" animation of holding the weapon (Sequence tab), for example: ACT_IDLE_SHOTGUN
Center the model around the gun, keeping head and lower torso in view.
Take screenshot.
Image is 256x256 pixels
Save in png format.
Open VTFEdit.
Import the png file.
Export as jpg file.