Only powders I can think of that may be of use for you are the W872 (#239) or AA8700 (#232). Slowest commercial powder I have on a burn-rate chart is the SP 13 Vactan (#246). Only one I've seen recently is 50BMG Hodgon (#244). I've loaded the W872 for the M103 Vulcan 20MM with the 1500 gr. blue tip practice round. 20MM is the largest cal I've loaded...so not much help there.