Sam did not say what kind of load he was using except it was behind patched RB. This was a test about accuracy, supposedly. In the same book he tells about a test he ran to see how much abuse(heavy load) you could give a gun before it blew up. He ran this on three guns. One gun was a .58 cal with a one inch across the flats barrel. He started with a 600 grain Minie and 100 grains of FFFg and worked his way up to 500 grains of powder. This didn't blow, so he backed up to 400 grains of FFFg behind two 600 grain Minie's. This also didn't blow so he kept the 400 grains of powder and shot three 600 grain Minie's. Everything held together. Finally, he went back to two Minie's in front of 400 grains of powder, except he left about 14 inches between the powder and the base of the first Minie loaded. That blew it up. This barrel was on a stand and was fired by pulling a string. He tried another just like the first and could not blow it up either until powder and Minie was separated. He then did a .50 cal Morse, then sold by Navy Arms. On this one he used both patched ball and Maxi balls. At one point he had three Maxis separated from the charge of 300 grains of FFFFg pan powder, no blow up. He finally put 200 grains of Bullseye smokeless powder behind one Maxi ball and that blew it up.

And I am afraid to shoot 100 grains of Pyro RS in mine.