The critical defense load is a .41 cal bullet over two 35 cal round balls. Unless the bullet has a hollow base, It's going to start yawing severely as soon as it leaves a smooth bore.
I've long wished that someone would manufacture rifled .410 barrels for common platforms (rem 870, Mossberg 500 etc.). Not so much for deer or defense, but because I could make up super cheap roundball slug loads that would be a lot of fun for plinking and small game/varmint shooting.
Even better would be if someone made a .410 die set for a single stage mettallic press so I wouldn't have to buy an expensive dedicated shotshell press for .410.