IIRC no once the Rifle is NFA it is always an NFA Weapon due to the rifle being engraved and serial registered as title 2. i could be wrong though, Im sure someone with a better knowledge will be here soon enough.
A suppressed rifle with a 16" barrel will not be a NFA SBR but the built in suppressor is still regulated by the NFA and is subject to its laws and taxes.
Likewise, a rifle with a 12" and a permantly attached suppressor makeing the barrel & suppressor length over 16" is also not a SBR even though the barrel is really only 12"
Most weapons can be taken out of the NFA registry and "un-regulated" per NFA if it no longer meets the NFA criteria. For example a SBR can be changed back to a title 1 rifle, a SBS can be change back to a title 1 shotgun however the rule of thumb is that MG receivers cannot be made back into semi-autos.
I am sure amafrank can be much more specific on the MG part then me.
Check the paperwork. If the suppressor has it's own serial number and it was registered that way without the serial number of the weapon then you can take it off and use it anywhere.. Then put a standard barrel on the lower