If you found yourself in a self-defense situation and you were able to get your BATFE registered SBS out of the trunk and load some shells into it and still have enough time to defend yourself, then your shooting would be legal as long as there was nothing else you could do but defend yourself with that SBS.
However, if you were in a state at the time that did not recognize your SBS as being legal in their state, you would most likely be charged with illegal possession of an illegal firearm and have it confiscated.
What exactly are you trying to "plan out" with this question? In other words, how are YOU defining SHTF?