A minimal amount of lengthwise play between barrel and cap section is inherent in the design of these twist ballpoints and is not a defect. Press the barrel towards the cap and it will move by well under half a millimetre, then spring back of its own accord to where it started, without sticking in any position. The cause is the twist mechanism in the cap section, which needs a certain tolerance to extend and retract the refill and is not built free of play. The same behaviour is found on other examples in the range.
There is no need to send the pen in for service on this account. Quite different are a clearly perceptible wobble across the long axis, an audible click, or a refill that no longer extends and retracts reliably: that points to a damaged or worn mechanism and belongs in expert hands, since Montblanc does not supply individual parts and repairs go through the factory service.