Kicks a little????? I should think so. 90 grains of Pyrodex will be giving you about 1900 fps ! That would be a heavy charge in a .50, but in the larger .54, I should think the recoil would be most noticeable.
You don't need that kind of velocity shooting a PRB, Tiger. The Factory data manual indicates that your load is equal to more than 110 grains of BP!
I suggest you back that charge down to 72 grains, for 1679 fps. Its more than enough, and will put that 1/2 oz. PRB right through a broadside deer out to your 130 yards.( 72 grains of Pyrodex RS is equal to 90 grains of Black Powder. Your actual velocity may vary with barrel length, and the loading components you use. This data comes from Hodgdon, the makers of Pyrodex powder.)