If what you have is labeled MR3100, it is the old surplus IMR4831. If it is labeled ANYTHING else, then what I said doesn't apply. I used up that old gallon jug of powder almost 30 years ago. I know they changed the formula and I know nothing about the new versions that have the 3100 moniker on them. This info came from Bob Hagel, one of the greatest gun writers who ever lived. MR3100 was surplus IMR4831. Harpy said MR3100 in his original post, not XMR3100 or AA3100. I never used data from either of the latter two. MR3100 preceded anything else labeled 3100.
Craig