6-shot tube extension on an 870? According to ATF's study, that shotgun has no sporting purpose.
However, those can be added post purchase, even to an 1100 (I'd like to compare apples to apples in a loose sense here). A Saiga is specifically built to be a detachable magazine fed semi automatic firearm. A Rem 1100 is tubular fed. While there are bolt shotguns out there that were designed to be fed from a detachable magazine, they are more specifically "sporting use" as per the ATF. Just because the study makes no distinction doesn't mean the information will be used in an adverse way. They may attempt to use it as justification to render (as many paranoid people believe) the Saiga 12 a destructive device. On the other hand, that would be a mess to clean up and I don't think they have the budget nor the political backing that would provide the extra funding to perform a witch hunt on Saiga 12 owners.