Montblanc offers its rollerball refills only in the line widths F and M; a broader version is not part of the range. Anyone wanting to write a heavier line must therefore change writing system, to a ballpoint with a broad refill, say, or to a fountain pen with a nib width to match.
Ballpoint refills do not fit a rollerball. The two systems differ so markedly in length, diameter and the shape of the tip that the refills cannot be exchanged for one another, and the mechanism or cap construction of each body is matched to its own system. Whether suitable rollerball refills exist from other suppliers has never been settled; with ballpoints such alternatives certainly did exist, among them Fisher Space refills in a version for the Meisterstück. If in doubt, a specialist dealer will try a refill in the pen before you buy.