In 1876, in the rank of an army captain, he went to Serbia and took part in the Montenegrin–Ottoman War as an artillery battery commander.

In 1877—1879 Rapp edited Russkiy Mir, originally a conservative newspaper, which during his years at the helm became more moderate in its approach to the internal affairs.

As an essayist, Rapp often wrote on the subject of the raskolniki, calling for tolerance towards their beliefs and customs ("Raskol and Our Legislation", 1878-1879).

Under Rapp as editor-in-chief, Russky Mir received three warnings from the officials, the last of which led to a four-month suspension.

His best-known literary work was the book Captain Kirillych's Serbian Stories (Сербские рассказы капитана Кириллыча), published in Saint Petersburg in 1877.