i am glad you are o.k.- you can always fix or replace the gun.
when reloading, i try to #1 pay attention!!!!!!! i feel for you, but that 550 did not double charge that case- you did!!! quit blaming the machine.
second thing i try to do is use a powder and charge that will overflow the case and spill all over, making a big mess if it is double charged. alot of manuels tell you how much case capacity listed load takes. this is useful information for reasons listed above. just another safety precaution.
i learned to reload with a lee aniversary kit, single stage. my close friend introduced me to dillon, and i love their machines. while slick, they are no substitute for attention.
again, i am very glad you are o.k.. learn from it, and remember the lesson.