Martial Law was never declared during Katrina. That was a tip of the iceberg compared to true Martial Law. True Martial Law is when Civil authorities/government is unable to function effectively, so the military would take over in that situation. These would not be Federal forces due to Posse Comitatus, thus State forces aka National Guard would take over for the failed government. As far as the confiscation of firearms during Katrina, that was a blatant violation of Constitutional Rights. I've heard some horror stories from folks that moved out of New Orleans and are in Dallas now. Law enforcement was either non existent or grossly abusing their authority. The word cluster fuck doesn't even begin to describe the aftermath.
Just know if Martial Law happens, your Rights will be swept under the rug, for you have become part of an operation. You can be detained (not arrested, but you can be held for quite a while), you can be searched, you can be disarmed, and you won't like it.