To me the gun case is different to the gun cupboard. So the gun with the silencer may not fit in the case, but it could fit into the cupboard OK.
We know it does fit, because we have the photo (above), but Anthony Pargeter says 'easily', implying that Jeremy would do this routinely. Yet I can't see how you could fit the rifle with silencer attached in the cupboard in this way without some effort, including kneeling down, tilting the barrel at a 45-degree angle, before pushing it upwards into place. You would then have to reverse this process to remove the rifle, which I would think must have been awkward. The difficulty would be exacerbated if the small table with the radio on was nearby, as this would limit the extent to which one could open the veneer door. Bear in mind also that there would be other guns and boxes in there while this is being done.
Wouldn't it be easier, and more natural, to unscrew the silencer, then lean the rifle at the side within the cupboard and place the silencer in a box?