RM,

My Evans did not group either. That's the Voodoo. I've read that some B&S guns behave like normal double ball rifles. Mine did not. It never worked, until it did.

You should start with Sherman Bell's loading methodology and a 740ish grain Fosbery slug. As for diameter, not sure, but I'd guess you would take the long diameter of the oval plus the short diameter of the oval and divide by 2. Then use bullets of this diameter to .003" under. I'm guessing here, maybe someone knows how to calculate bullet diameter for oval bore rifles? RS figured out which diameter to use in his Colindian, but left it out of the article in the DGJ. He obviously wants to keep these things cheap.

I doubt you have a RB gun. Everyone was competing with the Paradox, which used an elongated slug.