Daryl_S:
Perhaps I'm a bit naive regarding slugs in shotguns, but you're going to have to explain to me how it is safe to recommend shooting 7 dram loads in 12-bore shotguns, most of which are proved for the 3 1/2 dram equivalent!

...also a 550gr round-ball at 1500 fps sounds like you may be tugging at the apron-strings of the principles of proof!

Can you disclose the published pressures for these loads too?

Maybe these loads are safe, but they sound a bit on the high side to me. There was a very big difference in build strength between the 12-bore rifle or bore-gun and a 12-bore shotgun, remember. Loads for the former would most likely spontaneously disassemble the latter!

Curl:
There have been a couple of different Brenneke slugs over the years, the early ones had a cork base-wad which detached at the muzzle but the later versions had rivetted plastic base-wads which remain attached all the way to the target. The later versions shoot better. Both types are solid lead slugs, not hollow like the Foster-style slug. I'll try to find photos.

Never heard of Brenneke sabotted slugs, but the company is innovative so it wouldn't surprise me. What is common in Europe often doesn't find its way 'down under', but we are usually quick to emulate Uncle Sam!