Well one member is trying to organise a run of .450/.400 3" doubles in Heym (new_guy) so I guess a possbility may exist for a limited time.

As for them in .416, one question would be how many doubles are currently being sold in .416 Rigby and .500/.416. The only advantage a .450/.400 3" in .416 calibre would give is lesser recoil and velocity than the .500/.416 and perhaps lesser rifle weight.