still legal (I think)
Both the Cobray M11/9 and the "imfamous" Tec 9 were both sold with slotted barrel extensions available as an option. I have also seen "solid" barrel extensions, which mimic the appearance (but not the function) of a silencer, sold as legal items fairly recently.
If you don't want to contact the BATFE, you might find a manufacturer for these items and ask them.