MIKE GEORGE says: The usual method is to crimp the magazine tube so that its capacity is limited to two cartridges.
There are several ways of doing this, and one of them is with a plumber’s pipe cutter fitted with a blade that has been ground to a rounded profile so that it produces a deep indentation rather than a cut.
It will also be necessary to have the magazine tube fixed into the action so that it cannot be easily screwed out.
This can be done by pinning, or with a squirt of the hardest-setting grade of Loctite threadlocker.
The job can be done by a gunsmith, and, when it is complete, the gun will have to be sent to a proof house so that the modification can be verified by their official stamp.
It isn’t really a DIY job.