John - thanks for the info and caution over burning rate change for H4198.
Dr.D - Trailboss is really suitable for loads developing 800-1200 fps. There is data on the Hodgdon site for Trailboss in the 458WM. Trailboss is a powder you do not compress. Despite it's bulk it is a fast burning powder.
You have another powder available in the USA that is bulky and a lot slower burning than Trailboss and this makes it suitable for higher velocities. With this one you can develop hot 45/70 type loads for your 458. Trailboss is around the same burning rate as Red Dot whereas AA5744 has a burn rate similar to 4198, which makes it better suited to higher velocity reduced loads. AA have load data for the 458WM on their site. Unfortunately it's a powder we don't have available here.
