I know this is super-old, but in case you are still interested, http://world.guns.ru/smg/rus/pca-41-r.html quotes PPSh mass with 71-round magazine as 5.45kg, and w/o as 3.63kg, from which trivial math and non-trivial assumptions suggest the magazine is ca. 1.82kg.
OTOH, https://ru.wikipedia.org/wiki/Пистолет-пулемёт_Шпагина says it’s 5.3kg with LOADED magazine, so go figure…