This isn't an interesting thread.
I have bought a (now banned AR SIG m400 and a Arsenal AK) before the ban.
I do not know if I have the paperwork for these rifles. How would I prove that they were purchased before the ban?
If you're being charged with a crime, the burden of proof is on the state to provide actual EVIDENCE of guilt. For instance, if the state shows manufacturers' serial number records indicating that rifle's was made after the FSA went into effect, you have a problem. If the state produces a 4473 or bill of sale showing you bought it after the FSA went into effect, same thing...you have a problem.
Absent real evidence provided by the state that you broke the law, the state hasn't met its burden of proof. It is insufficient for the state to say "we think you bought that rifle after FSA went into effect, prove your innocence." That does not satisfy their burden of proof. The lack of evidence of innocence is not evidence of guilt.