Yeah, I was kind of thinking that as well -- but then there's that sort of grey area regarding the buttstock. In order for this short-barrelled shotgun to be considered an AOW Class, it may not have a rifled slug barrel nor a buttstock. Having a rifled barrel would move it into the DD Class (Destuctive Device), and having a buttstock would move it into the SBS Class (Short Barrel Shotgun), both of which require the $200 tax stamp.
Although use of a rifled barrel would be more realistic if you were actually firing grenades, most Remington 870's available back then including those used in the films were smooth bore -- so as long as you used one of those to cut down for a pulse rifle grenade launcher it could not be considered a DD.
However, since the Thompson M1A1 / pulse rifle itself has a buttstock, and the Remington 870 is now attached to this rifle with a buttstock, does that mean the Remington 870 now sports a buttstock by association?
I know that what is being attempted here is not standard weapon modification practice, and this would probably be a question better asked of the ATFE themselves. Regardless, I think this is a perfectly responsible question to discuss -- especially since some of us are legally abiding gun owners, some of which might be interested in the idea of such a build.
