GRIZ... I load .410 shells in 444 Marlin cases( with the head stamps knurled out )... deepened the rim on a couple guns just enough to take the fatter rim of the 444, & yet still will shoot factory 410 shells...
I can get 3" shot shell equivelent loads in a standard 444 case... ( I also load some .375 lead round balls, as well as shot )
does the 303 have a thinner rim, or do you just have a good supply of those cases ???
you might call making a Velo-dog case a "joke round" but I also make 40-65 cases from my 45-70 cases... & those are serious business