Marlin first developed the micro groove for their 22 rifles and thought it would be a good idea for the other calibers as well.

Maybe it works great for the 22 rifles with their lead bullets but like you have said,not so well for the cast larger calibers.

When I get my 336 back,I will check to see what rifling it has. I am quite sure it is cut rifling as I think Marlin came out with the micro groove stuff in the late 50s.

Wayne