You folks are absolutely correct that it may have been a dummy of a perfect shape and size painted as a live tactical nuke. Still, I have no doubts that the swedes saw something that either is or was made to look exactly like a nuclear warhead.
That does not remove the fact that the Russians WANTED to make everyone else think that they are cruising happily in other nations airspace with a live nuke.
Whether the bugger was then live or not doesn't make much difference, message was delivered, and at this day and age one could make an uneducated guess that the message goes along the lines of "join NATO and you'll be receiving these".
To emphasize my point: undoubtedly the Swedes know what they saw, and they have to assume it is a live nuke.
To admit the possibility: it may have been a repainted dummy or it may have been a live one, that's something we'll never know for sure. The message was delivered and is the main thing anyway.