Modern magnum caliber revolvers are high-performance instruments. The high performance has the side effect that the high-pressure gases at the cylinder gap can show signs of erosion on the top strap and the forcing cone. This erosion has different levels of severity based on the type and amount of propellant used in the ammunition.
True magnum loads utilize slow-burning, high-energy propellants, and lots of it. The cylinder gap between the forcing cone of the barrel and the cylinder allows high-intensity, hot-burning gases to escape. More powder, more erosion. Less powder, less erosion. Since this phenomenon cannot be avoided, the shooter can utilize moderate loads of target-type powder for practice to minimize the effects, and use the true magnum propellants when true magnum performance is required. Additionally, newer, high-nitroglycerine-level propellants – designed to yield higher velocities in magnums – create higher flame temperatures, causing erosion a bit sooner than some of the older, single-base propellants.
As the old adage goes, “sometimes you have to give to get.” With that, the amount of increased erosion is small, and over the life of the barrel, insignificant, considering the gain in velocity and performance obtained with these new propellants.