Model rocket motors are allowed to go to 200 degrees C. Estes virgin kraft thick paper cannot ever do that and besides BP burns at half the temperature of APCP. The polycarbonate cases Q uses (insert derogatory term I often refer to Aerotech as), are much thinner, have more than 2x the combustion temperature and are so optimised in material usage they actually made the case 0.006-0.01" bigger OD than useable.

Tech Jerry