It's too bad the Lyman BLACK POWDER HANDBOOK & LOADING MANUAL 2nd ed doesn't list anything except roundball loads for the .32 cal rifle.
The 45 grain .310 diameter roundball generates some pretty high pressures with its loads.
With GOEX FFFg, a 50 grain loads pressure is 10,400 PSI.
Using 50 grains of Pyrodex RS gives a pressure of 13,300.
Substituting 50 grains of Pyrodex P raises this pressure to 19,500 PSI.
Just to get a BS approximation of the effects of a bullet rather than a roundball, I am using the .36 caliber data. Although it really is not usable information, I thought it might be interesting to compare the effects of the two.
With 50 grains of GOEX FFFg going from roundball to a 125 grain Buffalo HP Conical raises the pressure by a factor of 1.129.
Substituting 50 grains of Pyrodex P, the raise in pressure has a factor of 1.312.
Substituting 50 grains of Pyrodex RS, the raise in pressure has a factor of 1.453.
If (and, it's a mighty big if) we use the pressure factors of the .36 caliber gun as the change ratio for the .32 caliber guns, using a bullet rather than a ball in the .32 cal would raise the pressures as follows:
GOEX FFFg: 10400 X 1.129 = 11742 PSI
Pyrodex RS: 13300 X 1.453 = 19324 PSI
Pyrodex P: 19500 X 1.312 = 25584 PSI
(NOTE: Black Powder guns can achieve this much pressure. The .32 shooting a roundball with 70 grains of Pyrodex P is shown to have 25,600 PSI shooting a roundball.)
My advice if your planning on using a slug is to go real easy with your powder loads. You may be in some real dangerous territory.