Longhunter, the rifle you describe was proofed 839 = August 1939 by the Zella-Mehlis proofhouse , ledger entry 245 of that month. It was made for Otto Bock, Kronenstrasse 7, Berlin, founded 1883, existed until WW2. Otto Bock is best known as the inventor of the 9.3x62 in 1905, but sold many smaller bore rifles on Mauser and Haenel actions in .25 Rem and 5.6x52R aka .22 Savage HP too.
The diameter marked by the proofhouses up to March 1940, here 5.5 mm, is not part of the cartridge designation nor the groove or bullet diameter, but the narrower bore or land diameter, marked in .1 mm increments. 5.5 mm and the maximum case length 52 mm is correct for the 5.6x52R aka .22 HP, CIP min. bore 5.55 mm = .218", min. groove 5.75 mm = .226", max bullet 5.76 mm = .227".
There are usually some very slight differences between imperial (inch) SAAMI and metric CIP dimensions due to different measuring habits, especially in pre-WW2 and pre-SAAMI (1929) cartridge and chamber dimensions. So try again with a factory cartridge or a full length resized case.