Just in case anyone else has a Viper and wants to know. I brought my shotgun to a gunsmith to make me an extension tube. After a little looking around, the extensions for Remington have the same threads.... The only thing he needed to make was a collar to slip over the existing tube so there was still tension when the extension was screwed on. Easy customization.
Can you explain in more detail and maybe post pictures. A collar? What Remington extension in particular? I am looking to do the same for mine.