You are fine with the S&W 500..... not so much with the 50 BMG
it is considered an offensive weapon here.( No kidding)
18 1/2 inch is the accepted min barrel lenghts although much to my surprise I did see the 16 1/2 inch barreled Rugers advertised here.
PS.
I did call the Firearms Centre here today, ( yep they work on weekends as well) and asked about those less then 18 1/2 inch barrels.
Much to my surprise this is not a criteria AS LONG as the firearm OAL is at least 26"
So those Ruger compacts with their 16.5 inch barrels are legal here. !!