I just received my Handloader Magazine No. 296, for June, 2015. In the "Propellant Profiles" section (pp. 16-17), R. H. VanDenburg did a review of the new IMR-4166 powder, which is one of the replacements for SR-4756, SR-4759, SR-7625, and PB. If you aren't a subscriber, the magazine should be on newsstands soon, or you can possibly get a copy online at: www.handloadermagazine.com
Dave