Herco was originally a shotshell powder intended for heavy loads. It falls between Unique and Blue Dot. I used a lot of it at one time in both .357 and .44 Magnum, but it would be useful in many calibers, especially magnum loads and loads with heavy bullets. I don't know if it is still made.
One load I remember well was a .38 Super load with 8.5 grains of Herco under a 124 FMJ bullet, I got an MV of about 1350 ft/sec out of it. Pretty hot for a .38 Super.