"These rounds cannot be used in a standard .410" smooth bore shotgun since the musket ball loaded at the mouth of the case is actually larger than diameter of the case itself. "

The round ball weighs about the same as the .410's loaded with 1/5th ounce slugs here. I seriously doubt they develop any higher pressures than any normal .410 shotshells.

Here, the velocity is toted as being 1,810fps iirc.

I have heard of guys shooting .44 mags. in their .410's.

I would not try that, BTW - grossly undersized in the head.

.444 marlin makes good .410 'brass' for tight patterning 1/2oz shot loads. It would also work for .410" bullets in a rifled choke - almost did that one myself as a test.

Couldn't get my buddy to offer up his .410 SxS for the trial.

.409" pure lead ball weighs 109gr.