When Christoph Funk advertized his then-new Mauser rifle chambered for the 5,6x61 vHSE, he noted a barrel length of 74 centimetres and a [IMHO doubtful] speed of 1130 m/ at the muzzle, with a 5 grams bullet of 26 mms length.
Load were a nominal (!) 3,5 grams of Rottw. Spez. Bl. P., which of course - being non canister grade - could be everything and nothing. My guess (only a guess) is that DWM used normal military Gewehrblättchenpulver as a start and phlegmatized it strongly. For every lot produced in the powder factory, phlegmatization was of course different, and hence the precise cartridge charge weight was empirically adjusted for this lot.