The difference between the two grades of Black powder is generally to be considered 15%, that is, you would use 15% less FFFg powder to gain the same velocity as a load with FFg powder. Now, because of the different rates of burning, FFFg powder is going to give you higher pressures no matter what you use. Wiht out a pressure barrel, I don't think we can tell you what change in powder will give you the same PRESSURE, one to the other. However, if you were to use about 82 grains of FFg, you should be close to the velocity given by 70 grains of FFFg. powder, all other things being equal. ( Same gun, same ball and patch combination, same loading procedures, same day, and weather conditions, etc. )
