I used first generation of Magnetospeed on a .50 DB rifle (indeed, .50 exotic calibre, not a .500) and on 12-bore DB rifles (and a number of SB). I've changed to Labradar in the meantime, but had to use Magnetospeed for the 12-bores as the Labrador got 'confused' by the felt 12-bore wads.
Problem with Magnetospeed is that you have to change between two shots if you'd want to do both barrels. This takes some time which -in theory- could influence precision of the second barrel ...
Beware of taper in barrels !
