The 1976 legialation had one curious effect.

Some guns were properly registered, some confiscated and destroyed.

But many more, once bought "just in case" or on a whim, were left hanging around, in the closets and barns (they still surface now and then). They were out of the light, out of legitimate circulation. Those who had them were normally just happy to get rid of them, for a bottle of alcohol or such.

So, when the first organised crime groups begin to appear in SU (just about at the time)? and until the widspread corruption opened the army arsenals for mafia in the early 1990-s, - they had a ready source of firearms in those "unregistered gun", which they usually sawed off to resemble a handgun.

Russian Hunting Shotguns