Why are Russian guns so crude

This is the tradition of gun making in Russia from Tsarist Russia to the Soviet Union to the present day. Because the quality of the soldiers in Tsarist Russia and the early Soviet Union were very low and not very good at using more complex firearms, they designed them to be simpler. For example, the Mosin-Nagant rifle, the AK47.

There is also the fact that Russia's natural environment is very complex and harsh, and guns designed to be more complex are easily damaged and difficult to repair. So you see, after the Germans hit the Soviet interior, the weaponry often went wrong, leading to defeat. But the Soviet Union's own firearms adapted to its own environment, but instead gained an advantage.