Try handloads with 'brush wads' and chilled shot.

That might get you south of modified.

Edit: Somehow I thought the Herter's shells were in 67mm cases, but they aren't, although they do wear the CIP logo.

I'm pretty sure it's the Gamebore shells that are in 67mm cases, although I have none on hand and can't check.



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