Büchsemann,
as far as I know, all modern Makers as Merkel, Krieghoff, Heym a.s.o. offer to shoot and proof their DR Drillings with slugs. I nearly can`t imagine, that a Bernhard Merkel Doppelbüchsdrilling, whenever much older, wasn`t proofed with Brenneke slugs as well...

But of course I may be wrong .

Gunwolf