Bullet head -- that's a new term for me.
After a quick web search it just looks like you tie the hair in with tips extending well beyond the eye and then fold them back (ala a thunder creek streamer) and tie the wing down, forming a thick hair head? Is that it?
In that case, I guess you are using the empty cartridge to ensure that the hair is all pushed back and in a uniform size/shape for the head?
Do not tell fish stories where the people know you; but particularly, don't tell them where they know the fish. ~ Mark Twain