They do that more so for the person that might have say a .40 caliber GMB and wants to shoot a .390 caliber roundball. Some of the small roundball barrels do not do well with large powder loads. So they lack in knock down power. And while I think a roundball even that small could kill deer at close range, it just is done because of the idiots that would be trying to knock one down at 100 or 150 yards. That is how it was explained to me by a warden when I asked the same question.

Now that .410 White you have is a game killer. That conical bullet would plant near any critter. But one reason they never interested me is it is basically a range rifle. Fun to shoot. Probably deadly accurate. But not legal for muzzle loader season. The same reason I did not purchase an Electra when they were on sale. In Wisconsin they are not legal in muzzleloader season. But they are in our modern rifle season. So it would be a range rifle toy.