LOL !

I don't believe, and didn't say one can safely propell a 500 gr "head" ??? projectile
at 2100 plus fps. The numbers you cite on ignition times don't mean much. stresses imparted and released during firing happen just as fast so what kind of case are you trying to make?

It is an unexplained phenomenon that slow burning powders can and do cause DR blow ups..
Matters not whether you agree with me but I'm making a case for the 3rd fastner.

Concider this theory:

slow burning powder not sufficiently expanding the case wall to the chamber wall and thus not effecting a proper gas seal..(Obturation) The result is the cartridge will then exert greater thrust.
This lack of seal is evident when the spent cases are black with powder residue and lots of unburned
powder down the barrel.

This condition is not unlike oiling the cartridge case and firing it.. like firing a proof load..
Do that enough times and fatigue will show it's teeth!

Note: This theory applies to all types of actions obviously, but due to the rather weak design
of the s x s action compared to an O/U or bolt action, will most likely have a greater negative impact on the former.

After all is said and done Most manufacturers oppt for the 3rd fastner..Hmmmm,I wonder why?

Think about it...