What do we know about 410 bore sizes?

I just acquired a K-20 barrel set in 410.

Barrels marked 10.2 mm which converts to .40157.

They do indeed measure .401 and .400.

The choke constrictions (Krieghoff screw-in) based on .400 bore seem reasonable, so this boring is intentional.

Is this common or is it just a Krieghoff thing?

Any clue why they underbore a 410 target gun?


"The price of good shotgunnery is constant practice" - Fred Kimble