[Who is John Galt?]

The Subcaliber flare gun device is designed for .45LC and .410 ammo and fits into a standard 26.5 mm flare gun. The insert is encased in a corrosion resistant adapter, and the offset barrel is positioned to make a perfect shot every time. Includes a BATF letter; flare gun and ammo not included. Proudly made in the USA by Runway Sub Cal.

Thank you! Alternatively, you can make your own adapters for various pistol calibers, using flanged &/or tubular bronze bearings, inexpensive pistol barrels, 12-gauge pistol caliber adapters, etc. The result is a single-shot handgun, the creation of which may or may not be legal in all jurisdictions, for all people, etc., and the operation of which would include indeterminate risk factors. But (as always ;^) "inquiring minds" etc., etc., etc...

[Kevmo]

Most of us don’t have tooling lathes so we buy these things on John Galt’s market.

One of the best rounds is the Personal Defense PDX round.
https://winchester.com/Products/Ammunition/Shotshell/Defender/S410PDX1