Shotgun cartridges

I may not be the first to think of this, but I don't know why I didn't think of it before.
Just been faffing about in the shed, trying to work out out what mid cap mags will fit (with a bit of persuasion) into hi cap mag cases, and there on the bench is a small bag of Shottie mags...... and on trying out, they will fit into most mags, most guns accept the bb connection, 30 shot per mag, trim the rim, slide into mag case as permanent or quick change (1 mag, 10 shot gun shells!)
They are about 20mm in diameter, so will slide into most gutted out mags, make up some sort of stop and platform to hold them in, maybe from plywood or plastic......30 round mags! A bit more realism for the WW2 scene.
Andy