I agree with Stallones however, when I "safey tested" my Manton I used 6 drams because that would have simulated a worst case scenario, that I had accidentally loaded two 3 dram powder charges into one barrel. It's a surprizingly easy mistake to make, in fact I did it once with another sxs ML. Fortunately, I tried to fire the barrel without any charge first.
BTW if you're going to use the gun you might want to invest in a wad puller (I forget the exact name) it's basically a small corkscrew that attaches to the end of your ram rod that you can use to unload your gun. You can find them in the Dixie Gunworks catalogue.
Steve