Raimey,
In your above discussion of velocity and aluminum casings, do you mean aluminum cartridge casings or aluminum gun case? I don't know of aluminum cartridges used at the time. Your rifle is chambered for 5.6x61R but the Waffen Scherell (photo above) is 5.6x61 (rimless, as it is a bolt gun), these are different cartridges. Please don't get the loading data for the two mixed up.
Mike