The modern Bics don't work.
I still have an old Bic that I used to size with. As has been mentioned the old boxed Prems stop at various stages down the Bic as they are larger pellets. A lot of the modern JSB stuff clinks right to the bottom.
I'm not convinced it's the heads and not the tails that cause the stop. The tails on most pellets are larger than the heads. I've taken pellets that I've placed in groups with the Bic ... then put them in again and tapped the Bic nose down ... and the pellets have then moved to the next mark. I'm convinced all I've done is sized the soft tails ... or corrected slightly out of round tails or corrected small burs on the tails. That may give the impression that it helps when shooting groups ... but I don't think it's selecting head sizes.
I've measured the heads of different Bic groups with a vernier and the heads are the same size. So I'm certain the Bic is just sizing or correcting tails.