I am using the core of 4B and use it, but since the hold is solid, the core will not be retracted. The balance when I have it is not so good, but I can use it feelinely because the core does not retract.
Up to now I used a 0.5 mm mechanical pencil, but it was hard to write because it was stuck in cardboard because of its thin core. Because this core pencil has a thick core of 2 mm, even a slightly soft cardboard can be written comfortably without stabbing. It feels good when you hold it in your hand and it is an item you want to use forever.