1) Do you have a stripped lower or any other lower assembly?
If you do, put a pistol buffer on it RIGHT NOW so that you can claim that is the lower that mates with the the upper.
I agree with the first post, if you have a rifle AR and a pistol upper you could be very, very fucked. BCG doesn't matter, they're not stupid.. Constructive intent means you can easily build it, if you have a full size AR (with BCG) then you could easily construct the SBR and thus you have constructive intent.
As I said, if you have another lower I would put a pistol buffer on it just in case so you can at least argue that is the lower that mates with it. But right now, I would get a lawyer and prepare for the butthurt.
Also call the impound and see if you can get your weapons out, tell them you're scared they will get stolen and can prove they are legally owned (bring receipts).