and the manufacturer told you absolutely squat other then to "only use the cartridge listed on the barrel"
I don't know to what you're referring, but, if it's in reference to H&R, then I can tell you they WILL give you an answer. At least they did for me.
In my case, I wondered about what pressure levels I could run my then new NEF(H&R) 45/70 buffalo classic at. You, (of course you know), have loading data in most manuals listed for 1, trapdoor springfield, 2, leverguns like the marlin, and 3, Ruger single shots and mauser conversions. The nice lady I got ahold of said levergun pressure/loading levels would work fine in the Buff. Problem is, the top loads with heavy bullets is just plain painful!
As far as Rossi goes, I don't know if they can/would give info like I got from NEF.