The pictures above are 0.38 pin-and-tube pen core and 0.5 bullet pen core respectively.
In fact, the difference between needle-and-tube type and bullet type is the thickness of the core.
Because the bullet type is self-contained, it can be applied to a very thick core, the thickest can be more than 1.2, which is very thick in the neutral pen. In our daily life, the vast majority of the needle-and-tube type is needed, because it is generally in the range of 0.3-0.5, and here is the edge.
It involves the difference between the semi-needle tube and the whole needle tube, which is the difference of thickness.
We can think that the pen core of the semi-needle tube is thinner, basically 0.3 specifications, and the maximum that the whole needle tube can achieve is more than 0.4. The difference between the two can make it easier for us to choose the pen core which is more suitable for our own use in different occasions.