hey DNS,
so far i have had success using conicals that are 380gr and are called "old style" (made from the 575213 mold) that i bought from Log Cabin Online.. they worked just fine.
recently i resupplied through TheGunworks and by mistake they sent me some of those 500gr .58cal conicals.. they are way way overkill and a waste of lead, if you ask me.
the problem is that this howdah is limited in the upside of just how much powder you can safely use..
when i emailed this very same question to d-pedersoli they said: "...We suggest to load 25 grs. of FFFg, .575" round ball and 010" patch...I would suggest to stay close to the round ball and patch, which does not need a larger black powder charge, that may give an unplesant recoil...."(direct quote from 'gloria' pedersoli tech support.)
granted, the 25gr of powder is their lawyer's recommendation and i personally have already tested FFF up to 40gr with very managable recoil.. the danger is not that the barrels might explode, but that the stock itself might not be able to endure the constant shock of high powder loads needed to push that size conical.
i've had success with the lighter conicals and rb, so that's what i'm going to stick with.. if you feel adventurous, go for it ! ! and let us know how it goes.
have fun,
~d~ :hatsoff: