Your RP should be more able to handle stouter loads than any of the old breaktops. Faster powders (Bullseye, Red Dot, Green Dot, Unique, and AA#2) are best in the .38 S&W. My mildest load, which I use in the old breaktops, is 3.0 grains of Unique with a 145 grain .360 lead bullet (from Missouri Bullet Co.). You could safely go to 4.0 grains of Unique, and I also use that load in more modern revolvers. I also use 125 grain .358 bullets with 3.5 grains of Unique in the oldies. Even with a slightly smaller diameter, those bullets work fine for me.