I dontsee how this could be a double charge. Lets say the bullet was seated .360" deep. This would give you room for 21.4 grains of AA No 5. If his charge was 8.0 grains or more (his charge was 10.7 grains) he would see a double charge which would fill the case nearly to the mouth.
Believe me, relatively fast powders and soft alloys will raise presure, quickly.