Rugerjim,
Glad you got it out and chrono'd the loads. AND, that the girlfriend likes it.
Powder speeds are a kind of general rule thing. They are measured for burn speed in relatively open capacity. This is good in that you can compare the speeds of the powders that way. What isn't tested (or at least published), and reloaders need to PAY SUPER CLOSE ATTENTION TO, is that when the load exceeds the normal weight or capacity, the burn rate of the powder gets faster. When the burn rate gets faster the pressure goes up faster, which in turn makes the burn rate even faster. Double base powders in an open capacity burn test fit right in as far as speed with single base powders. What isn't told the the reloader is what the pressure is supposed to spike to. This is why you will often find slower double base powders maxing out with less than single base powders.
Any variation from a normal load from a book can spike pressures.
Some factors are:
-A magnum primer increases the burn speed of the load. -make sure you have the right primer and start at the bottom again if you move to a magnum primer.
-Sharp shoulders such as Ackley improved. The angle of the shoulder causes the powder stack to bunch up without flowing down the barrel. Ackleys work somewhat, but the pressure spikes they endure are phenomenal in a very small increase at the top charges. Ackley's work extremely well when downloaded a bit. both in case savings and consistency.
-Overloading, -goes without saying. People thinking they can always get 100 fps more with the same powder than what was safely tested.
Short throat -When the bullet hits the lands it slows down. It slows down more with less throat.
-Bullets seat out into the lands. You can do this, just know you won't be near "book max" when you actually hit max.
-Tight necks- when firing if the brass in the neck can't go anywhere it will cause a pressure spike. Either tight chamber neck or cases that are sized down and the brass in them is too thick, therefore tight, in the neck. A reloader needs to check and turn these necks if need be, so that they fit the chamber neck correctly. BTW, This would still be an issue for you as well if you made a 6-284 instead of a 6mm-06.