It's the mass of the potentially detonating components. Can a 904 drum explode, yes they can, BUT, it takes a serious effort to get there.
727 is bigger in diameter and much heavier than the 904. It takes about 13500 rpm to grenade a 727 drum. With a 2.45 first gear multiplying revolutions, a mid 5K mill can get you to critical mass.
A 904 drum takes about 24000 rpm to reach critical mass. With the same 2.45 first gear that's an engine RPM of roughly 9800. If you have a 2.74 it's ~8800 rpm to get there. Not too many mopars turning those type of RPMS.
With some of the R5/P7 engines making appearances in drag cars, a 904 being used may experience an explosion especially with a low gear set. Those engines put out some serious power and a 904 may not be the best choice unless it's a very light car..
