Again, what type of ammo are your using? As was posted, BP will cause the barrel to heat faster and more than most smokeless powders. What was the ambient temperature (approximately) that day? Were you in direct sunlight or shade? What type of bullets were you using? Jacketed or cast? If you are shooting smokeless powder, the type of powder may have some bearing on the heat generated. Some of the faster-burning, double-base (containing nitroglycerine) powders may produce more heat than a single-base powder. I seriously doubt you can shoot the barrel hot enough to damage the rifling, but if we have more information, we may be able to suggest things to decrease the heating.