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Bic pen? The smoothest ballpoint for drawing is pentel client with the India ink in it. Though its mainly suited for smooth gradient shading.

I use Bic pen for commissioned pieces and Biro pen for sketches on my sketchbook. I rarely use roller tip pen though I have it. I haven't used pental client before.

Bics are pretty versatile, they even come in rgb so its possible to mix colors together.

Yeah, you are right.