I tried the Monteverde BP Gel refill M43 and M44 with mixed results (hesitates at starting). This Pentel refill also fits the MB Legrand Rollerball without much modification (you need to put a small piece of eraser from a mechanical pencil to lenghten it slightly).

To make this work you need to get the Pentel RB refill and meassure against the Montblanc BP refill and chop of about half an inch of the plastic tubing of the Pentel RB refill so it is the correct size for the MB BP. I am sure other Brands will work - you need to experiment by buying a few refills but stick to ones with plastic tubes so you can cut if you need to. I have only tried this combination as I wanted a smooth fat wet line and the Pentel does the job for me. MONT BLANC 163 MeisterstaCk Classique Gold Coated Rollerball Pen With Pocket Holder 6Cc ,Black. The Montblanc Meisterstuck 149 became such a great inspiration and legend that other pens - Slimmer version of fountain pens, ballpoint pens and rollerball pens were introduced to match these pens to make the most famous of Montblanc collections -Montblanc Meisterstuck. The Ball on the tip is massive and it does not dry out if uncapped. Mont Blanc Ballpoint Refill - Blue Medium 105151. It is in my opinion better than the Gel Refills for BP made by Parker. I am not sure what the Hybrid Pen stands for but it writes like a RB and not a BP. This Pentel refill is meant to fit the Pentel K230 line of Hybrid pens. I believe Pentel do a 0.5mm line as well (medium). I use a Pentel KFR10-A0 Hybrid refill with a 1.0mm juicy line (Broad). I am not sure if it works on a Legrand BP 161. but you have to have the 164 sized ball pen with a Pentel RB Hybrid refill. I'm curious, is there any way to use a roller ball refill in it? I saw some other posts going the other way, where you can buy a converter to use ballpoint pen refills in a roller ball model, but not roller ball in ball point pen. I recently purchased a Montblanc Meisterstuck ballpoint pen.

Montblanc meisterstuck pen refill