One aspect of black powder is it burns at approximately the same rate regardless of heat & pressure, once it reaches ignition temperature. you can pour a small amount of BP out on a flat surface & touch it with a match flame & it will flash. Under these same conditions it is extremely difficult to ignite smokeless at all. Thus a BP load will have similar ballistics whether the temp is 100° or 0°.
The burn rate of smokeless on the other hand is highly affected by heat & pressure. The more it id heated or the more pressure upon it the faster it burns & the faster it burns the more heat & pressure it produces. Burn rates of smokeless are controlled by both shape & size of the granules & by deterrant coatings. Thus a flake of Blue Dot for instance is thicker than a flake of Red Dot so if one weighed out 20 grains of each there would be more exposed surface for the RD. Also the BD would be more heavily coated. In order to reduce pressure without affecting ballistics of the load it becomes necessary to go to a slower powder than normal for that weight of shot charge & muzzle velocity. A load of this type which performs admirably with F° in the 70's may "Flop" when in the 20's.
While in total agremment that pressure is pressure, the means of getting there is not always anywhere near alike.
Personally I would not take a gun I felt "Needed" loads as low in pressure as 5K & expose it to 10's of thousands of rounds a year. For the limited amount of shooting I would do with such a gun, half the pleasure is derived from using the BP it was originally designed for.
Then again, I don't have any in-line Muzzle-loaders with 3x9 variable scopes on them either.
