I am uncertain whether my reply addresses your query, or whether it will be useful.
I owned a .58-caliber Kodiak that came with a quantity of conical bullets. Pedersoli's loading data for both Kodiak 58 and my Safaris appear to be egregiously conservative - probably guided by liability concerns. My conicals averaged 630 grains. My starting load - which became my load - was 120 grains Goex FFg with CCI #11 Magnum cap. "How did I arrive at such an apparently excessive load as my 'starter?'" you might reasonably ask.
I took into account barrels' wall thickness, knowledge that while patched RB pressures seldom exceed 10,000 L.U.P., pressure will nearly double as bullet weight doubles, with pressure topping out under 30,000 L.U.P. in stiffest black powder cartridge rifle loads. So while I'm still in "by guess and by gosh" territory, I was comfortable testing this initial load remotely (rifle tied into an old tire and fired via a string). Beyond dirtying the rifle as it flipped over for the four shots (two for each barrel), no issues.
Two of us shot this load to see what we were getting into. Recoil was acceptable - significantly less than my Safari loads, which need not be discussed here. Accuracy per barrel was astonishingly good. And best of all, barrels grouped within four inch cone at 75 meters using one rear sight. I am confident that had I kept the 58, I would have been able to tweak the loads into tighter grouping. When loading the conicals, I discovered that process was too slow for me to consider attempting to create any method of speedy reloading. Two shots is what I get. Any reload will take as long as it takes. Period.
What I do not know is the identity of the conical I shot. I kept several, the idea being to take the average of their dimensions and have a custom mold created if I could not find such a mold off the shelf. I no longer have the 58, but bullets are in storage.
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Unless you choose to shoot from each barrel a severely reduced load that allows you to retrieve fired bullet intact for measuring, your easier method is to measure - telescoping gauge, small hole gauge, or inside micrometer - bore diameter of each barrel. The bullet to be used would have a base driving band about .005 inch under bore diameter, bands increasing diameter until front driving band would be about .001-.002 inch under bore diameter. Using pure lead or 30:1 alloy, ignition will bump the bullets to fit your barrel - that is, your tinkering will be to improve accuracy and group by adjusting powder. And the undersized base band will allow you to load fouled barrels and thumb start bullets into muzzles for alignment.
Hope this helps.
