per Lyman, a 58 PRB, 28 inch Zouave barrel at 100 grains of FFG Goex gives muzzle velocity of 1215 ft per second. same set up with 120 grains of the same gives 1322 ft per sec. at 70 grains there is 998 ft/sec.
The point of diminishing returns is reached between 80 grains and 90 grains. By diminishing returns, I mean that for every ten grains increase in powder the muzzle velocity increase becomes less and less. ie for every ten grains from 50 grs to 80 grains, the velocity increases by approx 95 ft per second. But from 80 grains to 90 grains the increase is only 63 ft per sec. from 110 to 120 grs the increase is only 53 ft per sec. This occurs because for every ten grains of powder, the powder takes up more and more room in the bore, meaning the PRB has less barrel to travel to the muzzle. In addition, the additional powder is not fully burned before the ball exits the muzzle, resulting in increasing muzzle flash. So while the actual velocity continues to increase, the actual efficiency of the increased powder goes down. Even though the PRB velocity increases at a decreasing rate, the breech pressures continue to climb at a steady rate. The result is that risking dangerous breech pressures in higher powder charges actually results in little velocity benefit, and even less increase in energy retention at 100 yds.
Don't fall for the Popeye syndrome. More powder is not better.
