There is a difference between IMR and H. Some will tell you to never substitute which I strongly believe in one kind is one kind and DONT ASUME. But if you looking in the burn rate table in the reloading manual they are side by side. The difference between the two is H is coated with a micro surface so in different conditions it burns the same, where as in winter example IMR burns a fraction slower, and summer faster. For extreme accuracy you will notice but for fun and more then likely open sites on a 45-70 you will never notice. Plus most of the time you should not be pushing cast boolits as fast as jacketed, as well my 2 cents my shoulder likes slightly slower, and the brass lasts longer so cheaper.