Dictionary of Spoken Russian/Russian-English/Section 8

§8. ABBREVIATED NOUNS

Abbreviations, such as СССР (for Союз Советских Социалистических Республик "Union of Soviet Socialist Republics") are pronounced by the names of the letters, with stress on the last: [es-es-es-er], and are indeclinable nouns. They are M when the name of the last letter ends in a consonant; when the name of the last letter ends in a vowel they have the gender of the noun in the full expression: for instance, ВКП [ve-ka-pe], for Всесоюзная Коммунистическая Партя "All-Russian Communist Party," feminine because партя is feminine.