You can not fly with BP on a commercial aircraft in the USA. It is considered an explosive by air. For ground transportation you can reclassify it to a flammable solid for sporting purposes.
It has never been legal to load BP in cartridges for the sole purpose of flying with them to get around the regulations. I know people have done it, but it was not legal.
I am currently hoping to get my Special Permit from the USDOT renewed. It allows me to fly with BP on bush plane (which is a commercial aircraft) in Alaska. The FAA is not really wanting to grant the renewal.
I just retired in May after over 32 years with the USDOT as a Hazardous Materials Program Manager/Special Agent for the Federal Motor Carrier Safety Administration.
Fleener