Dave,
You might try 444 Marlin cases, they work well in my 410 pistol, loaded with polishing media for carpenter bees. They might work with bullets(45cal) in the Contender. Never try this in regular 410 intended for shot. If a 444 loaded with 45 bullet won't chamber, try seating the bullet "out" with lube groove exposed and running loaded case into 444 sizing die, to bottom of bullet. This approximates heeled bullet and works well in my 43 Mauser with .452" barrel( cast bullets only).
Mike