I have always used tube ТУТ нг 6\3 made by KBT. The tube from my latest purchase failed to be stretched over a connector.

The tube should not be stretched! Before shrinkage, the tube should be easily put on the surface that needs to be insulated. The diameter of the connector or any other surface should be min 20% less than diameter of the tube. The sizes of the tubes indicated in the catalogue are nominal.
As a rule, KBT Tubes with shrinkage ratio 2:1 have a diameter from 6 to 6.5 mm.