I don’t have a K Mauser, but an even rarer Schmidt & Habermann, Suhl, model 21 in 6.5x54 K Mauser. As you see, it is claw mounted with a Zeiss 2.5x “Zielmi”scope.

For several years I have been looking in vain for a proper die set. So I make cases from once-fired Sellier & Bellot 6.5x57 brass this way: Shorten cases to 53 mm (they grow longer in reforming). Anneal necks an shoulders. Resize case body by pushing them all the way into an 8x57 IS Lee die without expander. Set shoulder back in a shortened 6.5x55 SE die until bolt closes. Fireform. Load by neck sizing only in 6.5x55 die. Seat bullets using 6.5x55 seating die.
Photo shows, l to r: 2 original pre-war loads by RWS and DWM, my handload with a 120 gr PPU .264” bullet, 6.5x54 Mannlicher – Schoenauer for comparision.

BTW, contrary to popular belief the 7x57 Mauser M93 case is not a necked down 8x57 M88 case. The Mauser designed 7x57 case has a head diameter 0.1 mm larger then the 8x57, designed by the Prussian rifle testing commission. The base diameter of the 6.5x55 SE is even larger than both. The base of the 6.5x57 is 11.90 mm = .468”, the 7x57 is 11.99 mm = .472”, while the 6.5x55 SE is 12.17 mm = .479”. This precludes reforming 7x57 or 6.5x55 brass to the smaller 6.5x54 Mauser, let alonr the even smaller Mannlicher – Schoenauer case.