I don't think Russian SSR and the USSR were synonymous terms for anything but for informal colloquial usage.
The Russian SSR had only 51% of the population and 77% of the land of the Soviet Union.[1] Both the Belyrussian SSR and Ukrainian SSR were United Nations members states and constituent states of the Soviet Union.[2] After the breakup of the Soviet Union, some scholars have referred to Russian Federation as a rump state.[3]
[1] https://en.m.wikipedia.org/wiki/Republics_of_the_Soviet_Unio...
[2] https://history.stackexchange.com/questions/506/why-did-the-...