You are right when comparing the burn rates of powders suggested for cast bullet loads in rifles, the powders are considered fast for a rifle when compared to the typical jacketed bullet driven at 2800 fps or more. But they are much more suitable for cast bullet loads fired at 1100 to 1400 fps.
As for the pressures, powders like Unique up to 2400 do have a faster or sharper pressure curve but if you look at some of the peak pressures for those loads, they will be much lower than the rifle pressures using jacketed bullets and 3031 or even 4350.
Cast bullets are at their best when they are started into the bore quickly with enough pressure to cause the bullet to try to swell, but can't because it is already in the bore. That pressure also causes enough pressure applied to the grease grooves of the bullet to hydraulicly press the grease from the grease groove onto to bore, therefore the bullet lubricant is applied to the bore under pressure. This helps greatly to eliminate bore leading.
Too slow a start or too little pressure and leading can occur from flame cutting of the bullet or from less lubrication as the bullet goes down the bore.
There are a lot of cast bullet shooters that use Red Dot, 700X, Win. 231, AA2 and Unique with cast bullets in rifles. Unique being the slowest of those powders. These use light powder charges and sometimes are not as accurate if the powder position in the shell case is not kept fairly consistant when fired.
I have used 11.0 grains of Unique behind a 200 grain cast, gas checked bullet at .314" diameter in my 91-30's with a Remington 9-1/2 primer. The load is very accurate, no pressure problems and no leading. Velocity in the 91-30 tested ran 1235 fps and Extreme spread was only 24.7 fps. I call that a very good load that can be shot all day with no wear and tear on the rifle or the shooter. I usually load my cast bullet rifle loads to 1100 fps to a bit over 1400 fps, with 1200 to 1300 fps my favorite range.
I shoot just about every caliber military rifle made. My most used powder is Unique at 10.0 to 13.5 grains for 308, 30-06, 303 British, 7.65x54 Argentine, 6.5 Jap, 7.7 Jap, 6.5x55 Swede and 8mm Mauser. Mostly around 11.0 grains. There are others, but I think you get the idea. 2400 and Surplus WC 820 that acts much like 2400 are my next choice. The bullet weights vary from 180 grains to 215 grains.
There are shooters that like to push cast bullets to their limit using rifle powders, but my way of thinking causes me to think messing with the problems of leading with higher velocity cast bullet loads isn't worht it. Then I would go to low velocity jacketed bullet rifle loads. Thats me.
