I was looking over a stack of older photos and found a round marked DWM K 581 K, but I didn’t see a listing for it. What is the round?
According to Datig/Scranton Cartridges 1896 - 1956, basically a checklist of DWM numbers with case drawings, 581 is a “Huelse zur Bergmannpatrone Kal. 7mm.” 581 E is the same but “Kal. 7mm Stahl,” which probably means steel case. The case drawing looks like yours. Mouth, 7.71mm; Head, 12.60; Rim, 12.57; Length, 39.1.
This cartridge was used in the second half of the thirties for experiments that led to the assault rifle ammunition. A Walther selective fire short rifle fired this cartridge (according to Otto Morawietz).
Not a lot of info out there, but here is a case, although damaged so the dimensions would not be 100% accurate