I don't have any Ramshot Magnum powder but I do have a couple pounds of Retumbo leftover from my RUM days. Retumbo sits right next to Magnum in Hodgdon's burn rate chart, so its gotta be close. Magnum sits between Retumbo (slower) and H1000 (faster). It seems unusual that such slow powders work in this size case, but the bore is relatively small and 140gr bullets rather heavy for caliber. Problem is I find no loading data for Retumbo in the 6.5x55 so I'll be flying solo in uncharted airspace. Should be OK to start with 50gr if it will fit in the case. I can get a lot more stick powder in a case by trickling it with a little vibration applied, and I don't mind compressing stick powders a bit. We'll see how it works out.
